Chevrolet Repair: throttle problem, chevy dealership, intermittent fault

Question
I own a 2001 Chevrolet Impala. It now has 78000 miles on it. The problem is that when traveling down the road and coming to a intersection or stop sign ,you let off the gas pedal and the rpms stay up and the car maintains same speed ,will not return to idle condition . was in the repair shop at a chevy dealership 2 times for this problem without being repaired ,now warrenty had ran out .and i still have same problem the dealer ship said they cleaned the throttle plate  2times and should have fixed the problem . the engine is 3800 6 cylinder.

Answer
If cleaning the throttle plate has fixed the problem in the past this is the most likely problem that is happening now.

This problem is very common on fuel injected vehicles and the plate should be cleaned at every oil change.

Another possibility is that there is an intermittent fault in the IAC valve.