89 V6-191 3.1L. Stalling & Surging

I have a 1989 Pontiac Grand Prix V6-191 3.1L. 158,000mi That I just purchased, it ran fine during the test drive and for about a week after. Sometimes it runs fine but other times it hessitates and stalls at low RPM and surges, fluxuating from 300 RPM to 1000 RPM. I have not yet done a full tuneup on the engine. It seems to only do it after it's warmed up and not when its cold. I see that many people have this problem and I don't wanna be a parts changer and spend anything more then I have to on it. I only paid $700 for it and I expected problems but not sure where to start with this one. Thanks for your help!

Automotive Recalls and Technical Service Bulletins for 1989 Pontiac Grand Prix V6-191 3.1L. http://www.alldata.com/TSB/48/89480322.html