The symptom is most usually caused by a sticking IAC, which I see you have already replaced. I would be suspecious that you have a wiring problem to the IAC from the PCM, or possibly a bad PCM. I'm not sure which sensors have an impact on the IAC circuit, but the PCM may not be getting the information it needs to make the IAC function. Another posibility (but one I have not heard of before) is that the passages from the throttle body to the IAC are plugged so that it cannot function.
I would start by removing the IAC to see if the passages are plugged. If they are OK, I would have the system scanned to see if the PCM is commanding the IAC. If you are, then I would check to see if you are getting a signal to the IAC. If the PCM is not commanding the IAC, then I would check to see what sensors may be off enough to keep it from working without setting a check engine light.
If you get to the point that you cannot go any further yourself, you have little choice but to take it to a shop for professional services. Good luck with it.
