1989 BMW 325i, Check Engine sign, engine hesitation: what is it?

This has never before happened with this car; the car was hesitating very badly today after driving about 10 miles, and although it got a little better the rest of the day, it still hesitates a little from time to time, and the check engine sign comes on for a second, then goes away - comes on for a second, then goes away for a few minutes, then comes back on again, as the engine hesitates/lurches for a second or two, then runs okay again. This summer it was parked for six weeks at my mechanic, who worked on other things (rear wheel bearing); but this car needs to be driven regularly, although not too much, since it already has 175,000 miles on it. Once I picked up my car a month ago, the car was stalling/ hesitating when starting (this also had never happened before); the mechanic told me to add STP fuel system cleaner, twice, which I did, and that seemed to take care of it, and it's been running fine the last couple of weeks. I don't know if today's problem is related to the earlier one. Please help me! What could this be?