My truck idles way high and won't idle down.

My truck idles way high and won't idle down after driving at highway speeds for a while. I picked my wife up and the airport and when I got there it was idling at about 2300rpms. I drove about 25 miles home at highway speeds and when coming to a stop it is still idling high. I have tried cleaning the throttle body and have replaced the IAC valve. I'm stuck. Is there another electronic gizmo that goes bad or anything that happens like this to other Troopers? Seems if it were a vacuum leak it would do it all the time but sometimes on short drives it will idle ok. Thanks for your time, Wade