1988 Ford Escort Idle Speed

Why would the idle speed control motor reset to fast idle after the car warms up. No codes in computer and it only happens after the cars is fully warmed up. If I shut the car of within say 5 minutes of starting it restarts fine, after that when I start the car the idle is about 500 rpm high like fast idle. I have noticed that when I shut off the car the idle speed control motor gets a signal to extend up. Also, every now and then the car when puliing up to a stop sign the idle stays high for about 10 seconds before lowering down to the normal idle.