1990 740 Wagon, 16 valve engine, auto trans. 168K on the clock.

We've had the car for about three months. No issues. Today, my wife is driving to work, about 3 miles across town, never above 30 MPH, couple of stoplights along the way. That's pretty much how the car is driven every day. We been using regular (87, I think, ) no knocks, no pings and the car is right peppy for a Volvo.

Halfway to work today, the check engine light comes on, no symptoms apparent. She parks it for the day at work, then on the way home, starts the car, no check engine light. Halfway home, same thing, light goes on, along with a slightly rough idle.

That was about 2 hours ago. I just went out to start it, no check engine light, idled for about 2 minutes in the driveway. Running a hair rough at idle, smooths a little but not all the way when the A/C is turned off. Tach needle bounces up 100, down 100, between 550 and 750. In gear, drops another 50 PRM, but still bounces up 100, down 100.

Oh, and it's hot out there, about 96. This weekend, I'll do the usual stuff, clean the TB, check for vacuum leaks, etc. If I can't get it to duplicate the error, I'll go blow it out on the interstate for a while.

So my question is .....

With the check engine light going on, then off again, will the ECU store a code? Or does the light resetting itself also result in a clearing of any codes that might have triggered the light?

And any thoughts on what in particular might cause the on again/off again light?