1997 Buick Lesabre 2500 RPM idle speed

Engine Mechanical problem
1997 Buick Lesabre 6 cyl Front Wheel Drive Automatic 127xxx miles

i have a 1997 buick lesabre which I picked up a couple weeks ago, and it has a fast idle problem, the car idle's at around 2500 RPM sometimes. Its unpredictable. Sometimes when the car is started it will idle at a normal speed (around 700 rpm) and at other times it developes a fast (2500 rpm) idle, and eventually drops to a normal idle speed. If the car is started and it goes into a fast idle, you can unplug and plug back in the throttle position sensor and it immediately goes to a normal idle and doesnt fast idle again until the car is restarted. I have replaced 2 TPS sensors, the idle air control valve, the MAF sensor, and the coolant temp sensor. I also took the throttle body off of the car and took it completely apart, cleaned all passages and throttle plate, also IAC passage. No vacuum leaks either. I have also hooked it up to a snap-on computer dignostic, and there are no codes in the history except a TPS whenever I unplug it. No loose connections in the harness and no breaks in the wiring. Was looking for some insight as to what might be the problem and if this has been encountered before by anyone. Thanks for your help and advice.