[fdunford]

Sounds like a well hidden problem. You hit the obvious already like the IAC.

I am thinking it may be worthwhile to take the TPS (Throttle Position Sensor) off and ohm it out. If working, it will slowly sweep from a low ohm reading to a high reading. If there are problems with it, the reading will suddenly jump to a high/low reading.

if you have access to an ohm meter you can do this easily, thus, no need to change the part without verifying it as bad first.