Does it start doing it after the car warms up? If so, you have a faulty IACV or the wiring. You mentioned that the IACV had already been replaced. I've been on the same boat where the IACV that I purchased used was also bad, and the problem persisted until I installed a brand new IACV. Also, I had a similar issue about 20 years ago with my 96 GSR. Actual ECU was the culprit at that time.