Car shakes and the service engine soon flashes on/off

Check for a vacuum leak like hoses broken or off etc. same for plug wires if you can see them then check fuel pressure with a gauge auto parts rent it. See link.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

First of all, way too many people, including some mechanics, still believe there are only diagnostic fault codes to be read when the Check Engine light is on. That is absolutely not true. There are well over 2,000 potential fault codes that can be set. Only half of them refer to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. The light does not turn on for the other half of the codes.

Those fault codes are the only thing the mechanic has to go on when looking for the circuit that needs further diagnosis when the problem is intermittent and not currently acting up, as in your case. If fault codes were not stored in the computer's memory, what would be the point of having them?

The next thing is you can tell the relative severity of the problem by when the Check Engine light turns on and off. By far the most severe problems are those that cause the light to flash on and off. You are supposed to stop the engine right away. Too much unburned fuel is going into the exhaust system where it will burn in the catalytic converter and overheat it. Continuing to run the engine could turn a minor problem into a very expensive one if the catalyst melts and the converter must be replaced.

There has to be a fault code stored in the Engine Computer unless your mechanic erased it. Most likely it will refer to a cylinder misfire, and given the flashing Check Engine light, it's a good bet the cause is spark-related. With no test results or fault codes to go on, the best place to start is probably with spark plugs and wires. When you have an engine running problem, please list which engine you have.

We do not have any listings for a 180SX, so we have no clue which engine you have. You need someone to actually open the hood and look in case we need to do more research. HMAC300 might be able to solve this without knowing the important details, but I am not that smart.

Dandy. I am guessing since you have not had the car very long that you do not know the service history. For that reason, I am still leaning toward replacing spark plugs first and reading the diagnostic fault codes. This engine does not use spark plug wires. It uses individual ignition coils, one on top of each spark plug. If spark plugs do not solve the problem, an intermittently-failing ignition coil is the next suspect. The way this would typically be handled is if there is a fault code that says, "cylinder three misfire, for example, (code P303), that ignition coil would be switched with one of the other ones, we will say with cylinder number five. The codes would be erased, then you would have to drive the car again until the problem occurs. The next time the fault codes are read, if the suspect ignition coil is the cause of the problem, it will be cylinder number five that is setting the code. It would be "P305 - cylinder five misfire".

The problem your mechanic is facing is this is intermittent, and there is no way to know for sure if it is solved. You have seen yourself that the engine runs fine at times. If the mechanic replaces a suspect part, and the engine runs okay again, how do you know the problem is solved? You and he only know it is not solved if it acts up again. This is very frustrating for car owners and mechanics.

Until I know more and have some test results to analyze, I will let HMAC300 continue on. I will offer some suggestions along the way If I have something of value to add.

I would do what Cardiodoc says, start with plugs and wires kind of sounds like a plug breaking down or a wire or both.

Nissan 3.5 engines are bad for intake manifold failures causing random multiple misfires. But there is a bigger problem. If the misfire continues the catalytic converter gets damaged and because of its location, the debris from the converter can get drawn into the engine and suddenly you start burning oil. By then it is too late and you end up replacing the engine. Hopefully in this case it is something like a couple bad plugs, but either way you need to get it fixed.

If the CEL has been flashing there has to be a code stored. Perhaps you need to visit a different shop with a different scan tool or even take it to a Nissan dealer and have them scan it. It will probably cost a bit more but it is still cheaper than a replacement engine.
Nothing against the one you have been going to, but it is possible his scan tool cannot "talk" to the computer in your car correctly.

A common misconception yet, even on the part of some mechanics, is the Check Engine light has to be on to read fault codes. As Steve W. Said, that is absolutely not true. There are well over 2,000 potential fault codes that can be set. Only about half of them refer to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. The light does not turn on in response to over half of the codes.

Without those diagnostic fault codes, it could take a mechanic weeks to figure out the cause of some problems because he would have no idea in which of the dozens of circuits to start looking. When the problem is intermittent, the only testing that has any value is that which is done while the problem is acting up. At all other times there is no problem which means there is no defect to be found. The fault codes are stored in memory so the mechanic can read them later and know which circuit or system needs further diagnosis.

As long as I'm adding to my wondrous story, I should add that fault codes never ever say to replace parts or that one is bad. They only indicate the circuit or system that needs that further diagnosis, or the unacceptable operating condition. If you'd read through the list of fault codes, you'd see that no part is even mentioned in the majority of them. The people at many auto parts stores will read codes in the Engine Computer for you for free, but they will usually try to sell you a sensor or some other part because selling parts is what they understand.

Your mechanic is right to not want throw random parts at a problem. There are times when we have no other choice, but you have to understand that you'll need to drive the car to see if the problem acts up again, then you are expected to have too return to the shop again. A better alternative that can help is to drive the car with a "flight recorder". That usually involves connecting an expensive scanner or some other piece of diagnostic equipment, and the shop may not be willing to let that out of their sight. Scanners with a "record" feature work best when the problem acts up often enough that the mechanic can make the recording on a typical test drive. New-car dealerships often have a smaller version that can be connected for the owner to take with them for days at a time. You simply press the "record" button when the problem occurs. A few seconds of sensor data is recorded for the mechanic to play back slowly later, then he can watch what changed. Because that data passes through the tool's memory, the recording actually starts a few seconds before the button was pressed.

It is also helpful to understand how some fault codes are set. Many sensors are fed with 5.0 volts and ground, (0.0 volts), then they develop a signal voltage within that range. The throttle position sensor has mechanical stops the limit its range of travel to 0.5 to 4.5 volts. If a wiring or sensor problem causes the signal voltage to fall outside that range, that is when the computer sets the fault code. Other sensors are limited electronically to that same acceptable range. Some sensors, particularly electronic sensors, can develop a wrong signal voltage, but as long as it stays within the acceptable range, no fault code will be set. That is when the mechanic has to try to figure out what doesn't look right. Some sensors have very little effect on engine performance, but some are critical and their signal voltages must be very precise. On most cars other than Chrysler products, the main sensor for fuel metering calculations is the mass air flow sensor. It sits in the fresh air tube, and a loose hose clamp or even a small crack in that tube can cause a noticeable engine performance problem.

Having multiple computers on our cars today makes finding which circuit to diagnose much easier. The problem is we have a lot more electrical problems than ever before because of those computers.