Sometimes when I drive at exactly 30 MPH it feels as if I am driving on a bumpy road

Hello, I'm Danny.

Yes, all of your observations sound correct. From sitting for awhile there could be a flat spot or deformity on the tire. After sitting did you properly inflate the tires to the proper p.s.i. ? The brakes if old in age could have slight warpage causing vibration as well. The clicking sound you hear while turning however is the CV axle.If the CV axle boot is ripped/worn/deteriorated will cause a loss of axle grease.When the CV axle starts clicking that is a sign of its impending failure.Here are a few tutorials showing what is involved with CV axles and there replacement.

https://www.2carpros.com/articles/symptoms-of-a-bad-cv-axle-joint

https://www.2carpros.com/articles/replace-cv-axle

Let me know if you have any further questions on this issue.Hope this helps and thanks for using 2CarPros.

One thing that was overlooked is the replacement transmission. It likely has the wrong type of fluid in it. You can verify this by observing the shudder never occurs right away after starting a cold engine. In winter, you'll need to drive a good three to seven miles before it occurs. In summer, perhaps as little as a mile.

The biggest clue is when this occurs, hold road speed and the accelerator pedal perfectly steady, then with your left foot, lightly tap the brake pedal. The shudder will stop for a couple of seconds, then come back.

This is caused by the lock-up clutch in the torque converter. It locks up to provide lower engine speed and better fuel mileage at higher speeds. At around 35 - 40 mph, or 30 mph in your case, it is under partial lock to cause it to engage slowly, for comfort. The transmission fluid doesn't have the right combination of additives to allow that to happen. The clutch grabs too aggressively, then engine torque breaks it free, then it tries to grab again. Once the fluid is changed, the clutch will be able to slip smoothly until it locks up solidly.

This clutch also doesn't lock up at all until the engine reaches a certain temperature. It unlocks when you tap the brake pedal in anticipation of slowing down to a stop. On most cars it will also unlock from just releasing the accelerator pedal to coast, and it usually unlocks at anything over about 3/4 throttle. Due to the characteristics of torque converters, when they are unlocked, they transmit twice the torque from the engine to the transmission compared to a manual transmission. When locked up, you lose that torque doubling. That is beneficial when pulling out to pass or when going up a steep hill. That's why it unlocks at high throttle.

Once you get above roughly 35 mph, the clutch will lock up solidly, so there won't be that shudder.

It is likely not causing damage. In some misguided attempts at solving a different elusive problem with my 1994 Grand Voyager, the previous owner had the transmission worked on three times, and the last time they left it with the wrong fluid. I've had this shudder for the three years I've been driving it. When it gets bad, I press the switch to lock it out of overdrive / fourth gear, until I get to a higher speed zone. For my van, it isn't road speed that is the variable. It's engine speed, and it just happens the engine speed where this occurs is what it's at when I'm going 35 mph. Shifting to third gear keeps the engine speed higher, and the lock-up clutch stays solidly-engaged at that engine speed.

Owners who pay a lot of attention to their cars will notice things like you're describing. One of the concerns with automatic transmission fluid is it has to be fluid enough in cold weather to flow freely through the cooler. Too much restriction there can cause damage when there is insufficient returning fluid to lubricate some critical parts. To address that, there were some cars in the early 1990's that we had to add a bypass valve under warranty / recall. That insured the fluid would keep flowing until it had warmed up enough to be sent to the cooler. What you perceive as transmission performance issues could be related to the same thing. Delaying up-shift points keeps engine speed up, and that keeps the transmission's front pump speed up. That helps pump the cold fluid.

There are other things that change related to engine performance in cold weather. Fuel in liquid form doesn't burn. It has to be a vapor. Higher engine speeds can result from the excessively-rich mixture they have to get in hopes enough percentage will vaporize to make the engine run smoothly. Burning more fuel makes the engine warm up faster, which reduces emissions.

Most cars today use some version of Chrysler's computer transmission controls, but older models used all hydraulic controls. Pressurized transmission fluid is switched on and off through passages where it pushes on the shift valves. Those valve can be sluggish in cold weather which causes late or delayed up-shifts. Once a shift valve moves, it directs fluid to a clutch pack which has to fill before it engages. That also is delayed when the fluid is cold. All of those problems have things designed in to help them run and shift smoothly under a variety of conditions including cold weather. We don't notice most of those things, except when it affects the "personality" of the car, as you've described.

I'm just explaining what you are likely feeling, and the most common cause. Mine shudders pretty hard at times. When it gets too bad, I adjust my speed or shift out of overdrive to lessen the vibration. The constant hammering between the engine and the transmission can't be good for the torque converter or the bushing it rides on, but in my case, changing the transmission fluid would double the vehicle's value.

What you might consider is having your mechanic use a hand-operated vacuum pump to draw as much fluid as possible out through the dip stick tube, then refill it with the right transmission fluid. You should be able to replace about half of the fluid that way, which is the same as when we do a typical drain-and-fill with a new filter. If it's nearing the mileage for a scheduled transmission maintenance service, now would be the time to have the filter replaced, and the fluid changed.

There's too many potential causes to make a guess worthwhile. A shift valve could be hanging up in some varnish build-up in the valve body assembly. The computer could be delaying an up-shift due to a glitch in the throttle position sensor's reading, or a momentary change in coolant temperature due to the thermostat cycling between open and closed, and cooled coolant is suddenly flowing in from the radiator. I've had the first two happen to two of my vehicles. The sticking shift valve cleared up on its own over a six-month period. The second one was found to be an extremely unusual sticking throttle position sensor.

I wouldn't be too worried about a simple delayed up-shift, as long as there's no slipping between gears. That would cause the engine to speed up while the car doesn't. Excessive slipping tears up the transmission's clutch plates and will lead to an early failure.

Slipping is when the engine speeds up too fast while road speed doesn't. Same as if you were driving a car with a manual transmission and you pushed the clutch pedal part-way down. Slipping causes the clutch plates to overheat and wear the friction material off. That leads to a total failure very quickly if you try to keep the car moving. You could consider yourself lucky to get 10 miles before the car just wouldn't move any more.

What you're describing is a simple delayed up-shift. That by itself is not really something to worry about unless it gets real bad. When this is just minor enough to be noticeable but nothing else, figuring out the cause can be almost impossible. We'd spend way more time than the cure is worth. If you find on occasion it up-shifts a gear at 35 mph instead of the normal 30 mph, for example, ignore that and keep on driving.

Shift points are calculated by looking at road speed and accelerator pedal position. Pressing the pedal further equates to harder acceleration, and the need to wait longer for the up-shifts to occur. Each of these functions, (road speed / pedal position) has a source of fluid pressure that rises proportionally. Each of those pressures are applied to opposite ends of a "shift valve". That's basically a short round metal bar that slides back and forth in a matching cylinder. The higher pressure pushes the valve to the lower-pressure side. When the road speed, (governor) pressure becomes higher than the accelerator pedal, (throttle) pressure, the shift valve slides to the other side, then notches in it allow fluid to travel to different clutch packs. One clutch pack releases, then the next clutch pack engages. That's the shift to the next gear.

Sometimes the bore the shift valve slides in develops a coating of varnish. That causes the valve to stick. That's when you need a little more road speed with its higher governor pressure to convince the valve to move. You're doing the same thing when you let off the accelerator pedal a little. That lowers throttle pressure which also lets the shift valve move easier.

Remember, this all has to do with shift points. That is completely different than slipping.

To me it sounds like the engine is misfiring at 30 MPH which is when the engine is trying to be the most efficient. When is the last time you had a tune up? Can you please shoot a quick video with your phone so we can see what's going on? that would be great. You can upload it here with your response.

This guide can help us walk you through the steps:

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please run down this guide and report back.

The delay in shifting can be because of the engine misfire. Is there a chance you can make a video of what the car is doing so we can see for sure what's going on?

This sounds like an engine misfire did you do the tune up with plugs, wires, cap rotor?