1989 Pontiac Grand Am

I have a 1989 Pontiac Grand Am. 2.5 engine. After driving for about 20-30 minutes and just before I come to a complete stop, the engine (car) bucks and then stalls. It will start rather easily but if you put it in gear right away it will stall again. If I wait 20-40 seconds it will drive fine. (usually). No idiot lights come on to indicate what I need to look at. I have replaced just about everything I can think of. I would love some ideas and or help. Thanks SAL