Engine stutters a lot.

When this problem occurs, does it happen right away or only after you've driven a few miles?

If you have to drive it a little ways before it acts up, suspect a plugged pickup screen in the gas tank. That's not related to how full the tank is, but having more fuel to wash around can delay when that screen collapses. Often it will relax and let fuel through again after sitting for a few minutes with the engine off. The problem is aggravated when the largest volume of fuel must be pumped, ... Which is during coasting. Once you're up to speed, the engine will run well at highway speed until you start to coast again.

If your engine has the fuel pressure regulator on the engine like my '88 Grand Caravan does, disconnect the vacuum hose from it and plug it. You might see black smoke from the tail pipe, but if the engine runs better, that's a clue to check the screen.

Logic would dictate something has come apart in the fuel pump assembly in the tank. My best guess is the pump does not need to be replaced but it will have to be removed from the tank to be inspected.

Ford had a similar problem years ago. I don't remember the years or models affected, but a rubber hose in the tank would dry-rot and crack. That would allow it to suck up air instead of fuel. The fix was to replace that hose.

I can also visualize that pickup screen being deformed and sticking up into the air at the top of the tank. There is a large air pocket in the tank when it is "full" to form a crush zone in case of a crash. That means that when your tank is three quarters full, it's actually about half full. Those tanks aren't real tall to begin with so that screen wouldn't have to stick up very far to draw in air. The screen is a lot bigger than on Fords. Yours will be about six inches long and two inches wide.

What I would do is have the tank close to three quarters full when you take it to the mechanic. They will have a tank with a pump to remove the gas so the gas tank is light enough to remove. After the repairs are completed, they pump the gas back in. Ask them to start by only refilling it to one quarter tank, then test driving it. Once the repairs are confirmed, they can pump the rest of the gas back in.

Here's an entirely different thought. Once the problem occurs, how much gas can you put in at the gas station to fill it? From what I can remember from working at the dealership, those tanks only hold around ten gallons. If that's about how much you can put in, you're actually running out of gas and the gas gauge is reading incorrectly. The float arm for the fuel level sending unit could be hanging up causing the incorrect reading.

Newer cars just have more expensive problems, definitely not fewer problems. It's why I refuse to give up my '88 Grand Caravan.

Overheating is not that common. You might look at the cooling fins on the radiator. If they are corroded and crumble like a rotten chocolate chip cookie, they aren't passing the heat to the air. That happened on my van many years ago. At first the engine ran a little warmer than normal if it was over 60 degrees outside AND I drove over 60 mph. Eventually it even ran warm when it was 30 degrees outside. Running the rear heater brought the engine temperature down right away. That was the clue the radiator wasn't dissipating the heat.

I can guarantee the cause is not the catalytic converter. If it is plugged, the engine will be very low on power all the time, it might idle uncommonly smoothly, and the sound from the tail pipe will be more of a hiss than the normal "putt putt". Problems with the converter have absolutely nothing to do with the level of gas in the tank. Tell him to drive the car home for the night, then back the next morning. If he has to walk to the gas station half way to work, he'll change his diagnosis real fast.