I had this exact symptom with a 90 SSE. Would not idle, but it would start and run fine. Anytime you would let it idle, it would just die.
Drove me nuts. Finally traced it down to a bad connection on the battery terminal. Cleaned up the battery terminals and problem disappeared. When it showed up again a year later, cleaned the battery terminals again, and it disappeared again.
I also had some wierd ECM related problems on a GM diesel that were resolved by cleaning the battery ground connections. Now I always clean battery and ground connections first.