The knock sensor may be doing it. I had a 91 Corsica, different fuel system, but it went bad. Driving or at idle it would surge real bad. changed the sensor ran great.
Don't remeber if you can test the knock sensor or not. It should be cheap if you want to try to replace it. Not hard to do.
How does your air filter look? When was the last time the throttle body was rebuilt? Kit cost about 30 or 40 bucks. It also comes with a FPR which could be part of your problem. Good time to clean your TBI too.
I would also check your timing.