Nice description of your issue, it helps to eliminate a lot if things. Just to clarify, does the idle only race when the car is in gear? You can try the "tap test" on the ECM to see if this changes the situation. When you replaced the IAC, did you also clean the bore in which it rides? An extremely high idle, especially an intermittent one, usually goes back to either the IAC, ECM or a vacuum leak. You are definitely approaching this issue with an intelligent attack plan, so keep us posted.