93 Pontiac Transport 3.8 fast idle

What are the possible cures for a very fast idle up to 2500rpm once engine is at operating temp? Occurs mostly in warm weather, rarely in winter, and
especially in stop and go traffic. Engine temp is always normal. This started happening about 2 years ago but is doing it just about every time I drive it now. I eventually get a check engine light which will turn off once I get on the highway for about 5 or 10 minutes. I finally read the code today which is 21 -- Throttle position sensor signal voltage is high. No other codes appear. I removed the idle air control valve today and cleaned the plunger and the hole and seat that it goes into. I was told to do that before buying a new one. They were quite dirty so I figured that might be the problem as it might not be seating properly or maybe seized a little. No luck! I checked all the wire connections that plug into the intake in that area for different sensors etc. and all look clean with no corrosion or broken wires. I not sure what is plugged in directly under the idle air control, but I figure it might be the throttle position sensor since the bottom end of the throttle shaft goes into it. If the signal voltage is reading high per the code 21 does this indicate a faulty sensor or is it high for some other reason like maybe a jammed idle air control plunger. I assume these two things must work together to adjust the mixture and thus the idle speed. I'm just trying to think logically here but maybe I'm way off base. Once the engine cools down for about 15 to 30 minutes the problem resolves itself and the idle is normal until it gets hot again. The heat seems to cause whatever it is to stick or malfunction. In the winter it rarely happens. I talked to a couple of local mechanics and they thought the idle control valve was the most likely culprit but I couldn't give them the diagnostic code since I had not yet read it. Any thoughts on this would be really appreciated. Thanks - Ken
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stihlwielder had written this in response to http://www.motorsforum.com/gm/93-Pontiac-Transport-3-8-fast-idle-37862-.htm :
My girlfriend had a bad throttle angle sensor in her '93 Buick LeSabre 3800. The check engine light came on one day and stayed on but there were no engine problems so we left it alone. One day the car started stumbling badly on the highway so when she got back I had the OBD1 code read at a local shop ($30). The reader said "TAS voltage too high". Concerned that a different part might be bad, I checked the TAS before replacing it ($70). It has 3 wires; the two outside wires are connected to a rheostat (like a volume knob) that changes the resistance between those wires. The check is simple: engine off, unplug the sensor, put ohmmeter probes on the two outside wires and slowly open and close the throttle as you watch the reading. Hers went smoothly for a bit then the reading jumped, then back to smooth before another jump. The jumps indicate a bad TAS. A good one should change the resistance slowly and steadily all the way through range. I hope this helps someone!
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