My first thought...air flow problem.
Might be is a air leak behind the AMM. Check for holes in snorkle or loose clamps. A split vacumm hoes something that could have just gotten wiggled upon shutting down.

Second, the throttle switch is not closing down all the way. Make sure it just clicks as it just opens and closes. The bore needs to be clean and tidy for repeitive operation.

You didn't say what year the car was.
You know about the idle control so, I suppose you are aware of the basic idle procedure. Its important to get the IAC centered in its working air flow range that's below a 1000 rpm's.

The throttle can't be held open (if any I say)to far nor can the idle mixture screw be used except when the wire is grounded.

Hope you found it. If not hope this helps!

Phil