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I had thought the same thing when mine had issues and logically replaced my temp sensor first. It turned out to be the fast idle solenoid that was bad and had a mind of its own. It would work fine one day but the next day it would race the engine to about 3/4 throttle. Just the same as the OP...

http://www.2040-parts.com/nos-ac-delco-14066255-solenoid-new-nos-gm-fuel-injection-idle-speed-control-i2329042/

Its called the Fast Idle Solenoid. No longer manufactured but still out there on eBay from time to time "$70.00+". It sets the idle up high when the engine is cold. Its activated by a temperature sensor in the top of the block by the firewall. Just keep it unplugged in till you find a new one...