Idle Problem Need help!!

I have a 98 grand prix with the 3.1 engine. I also have flucuating RPMS at idle it will just start to jump around (search) when vehicle is warm. It sometimes stalls when I put it into drive. I have done pretty much everything. Changed IAC today. that was 71 dollars. I have cleaned the intake and still no change. I am in desperate need to fix this because I am selling this car because I purchased another vehicle. When I purchased this car 2 years ago it started doing the same thing. 02 sensors were replace, and it was also tuned up, and now because of the cold weather in Winnipeg, I am faced with the same problem. Please help..
Should I change the Throttle positioning sensor? But this is like a Potentiometer, and should cause this to problem only part of the time. Do I change the O2 sensors again at a cost of 300.00 dollars???I'm confused!!
Crazyman:banghead:

Check any possible vacuum leaks at the intake manifold area, the brake booster and to all sensors like evaporative emissions purge, etc. TPS is a possibiity and maybe the MAF sensor. I'd run an vacuum test with an automotive vacuum gauge to see if in specs. 15-22 in hg.

Hmm.. Had my buddy come over and run a diagnostic on the car. Shows nothing! Everything from the TPS, EGR, MAP, IAC are all running fine. Seems to be intermitant. Must be electrical.. Maybe another tune up, but this was already done approx 2 years ago.
Still very confused!
Crazy