I'm guessing Fast355 asked about motor mounts because the engine could be shifting/rotating as the RPM increases/decreases. Yes, as far as I know they're fine. I had the transmission rebuilt recently and the shop looked at the engine and transmission mounts at that point. I'll double-check that to be sure.

I drove the vehicle again today. The engine did not stall/die like it did in the past. As I mentioned above, I've moved the ECM to keep it away from any moisture coming from the A/C duct work.

I did get a check engine light and it seems it always comes on at idle or when the engine is dropping down to idle. I don't recall that it every came on at highway speeds.

At one stop light the IAC clicked up several ticks and I could hear the sucking sound and noticed the increased RPM. Once again, I think the ECM is trying to compensate for a below set-point idle speed so the IAC overcompensates. The A/C was on at the time and that old A6 style compressor really puts a drag on the engine. Combine that with the variation I see in the idle speed screw setting and it's easy to imagine the ECM is doing some wacky stuff.

Any new ideas to consider?