1995 Chevy Caprice FAST IDLE

My 95 caprice, has a problem with speed of motor not releasing when you let off the throttle. It will go back to normal but slowly, The check engine light used to be on all the time till I found this sight. I read your ansers to a similiar problem, and a mechanic said there problem was the sensor on the water pump I changed mine and the light went out a cured my hard starts. Now after 6 months the light cames back, I replaced the same sensor and during cool morning im getting the problem I listed in the first sentence, but the light goes out after 15 miles and its back to normal running.