Based on your description, I would start by looking how long it has been since it had its last tune-up. We don't really use the word "tune-up" anymore, but it sounds like part of the problem is related to misfires. As a general rule when you turn the ignition switch off and restart the engine and the problem goes away, even for a little while, it is USUALLY not a mechanical problem, as in a broken part. It could be related to a worn or sloppy part that is allowing too much play, but what you described sounds more like electrical in nature.
It would take me all night to type what I know about the Check Engine light's behavior, and that wouldn't help you much. On Chrysler products once the light turns on, even if it goes back off, there will be a diagnostic fault code stored in the Engine Computer. Very often they don't get stored in GM computers until the problem acts up long enough. They call those "pending" codes, meaning the computer is keeping a closer eye on something it suspects is a developing problem. Often that problem never does develop. What is important to understand, and what your mechanic didn't describe very well, is fault codes never say to replace parts. They only tell which circuit or system needs further diagnosis. Without those codes we could spend days testing every conceivable part and still not be close to solving the problem. With the code you had before, that problem is not acting up now. That's why there is currently no code and it is why the mechanic won't know what to test or look for. He knows the circuit that had the problem, but the problem is gone now and there is no defect to be found.
When the Check Engine light stays on is when there is something for the mechanic to look for. Once the suspect part or cause is identified and fixed, he will erase the code and test-drive the car. If it really is fixed right, the code will not set again and the light will stay off. The problem is sometimes codes set after the computer runs a series of self-tests while you're driving, and certain conditions must be met for those tests to run. For that reason a problem that is still there might not be detected for many miles or even days.
Many dealerships have a sort of "flight recorder" they can plug in that you can take along for a few days. You press the "record" button when the problem occurs. Later they can play that back and look at what each sensor was reporting to get some clues. Since there's a computer memory involved, that recording actually begins a few seconds before you pressed the button, so you don't miss the event.
Also be aware there are some self-tests that some computers are programmed to do that can be felt by the driver. This is more common on cars that are only a few years old. I can't tell you what they all feel like or at what speeds they occur, but if they pass, that is recorded and often no other testing is required in states that have emissions testing.
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Friday, February 8th, 2013 AT 8:37 AM
