Welcome to the club!!
This seems to be a commom problem that plagues many S2 owners. So far, through my research, there isn't any one definitive correction.
I've heard that it could be a faulty Throttle Position Sensor, a faulty Idle Stabilizer Valve, or a vacuum leak. I've also heard that it could be a faulty DME relay.
I've cleaned some of the sensor connections, cleaned the throttle body, adjusted the throttle linkage and cleaned the terminals on the DME relay, all to no avail. In fact, the problem has gotten worse on my car. It used to only exhibit the symptoms upon a warm restart, but recently, it now rears its ugly head all the time.
It sure is annoying, dangerous, and unpleasant. It sure takes the fun out of driving the car.
I have an appt. next week to have the problem looked at (by a very knowledgeable Porsche shop), so hopefully I can post the cure to the problem (at least whatever it is on my car) sometime next week.
Jim