Try this ... disconnect the IACV electrical harness when this occurs and see if it stops. My engine runs much smoother and quiter with the IACV disconnected, mostly because my IACV is shot and the plunger is constantly part-open.
I never understood why they didn't just drill a hole in the throttle plate for idle. The volatage to the IACV doesn't vary as far as I can tell ... it's either on or off. Also, the harness only has two wires ... meaning there is no return signal, so it's not an intellignet system. So, why have it???
One train of thought is for off pedal emissions. When the car goes from full throttle to no throttle, a rich condition temporarily exists in the engine. With the IACV there, this can be controlled by allowing the IACV to introduce more air in this situation, thus lowing emissions.
I will attempt to remedy the problem in the coming weeks with a "home brew" solution and will keep everyone updated. BTW, with the engine running, unplugging the IACV will not throw a CEL ... I have yet to start the engine with it unplugged to check. I hope that my research this week will answer this an many other questions.