Wow you really have done a lot to hunt for this problem.

Yes, the intake manifold gasket might be a culprit, as well as maybe the throttle body and its idle position and its switch. Check there too, and clean it out.

Idle hunting is often due to IACV (you said you replaced) or the AMM. Did you replace again with a 016 AMM and not an older 007 or before? You need the same one.

The temperature sensor burried under the intake? That connection often goes bad, or maybe the sensor (see FAQ).

Ignition timing?

If the check valve is stuck open you would have a hard time starting the car when left hot for a few minutes, heat pressurizes the gas and sends it away from the front. Cheap but labor intensive to fix, I did mine. New fuel filter while you at it. Likely you should have done all that at time of replacing main fuel pump, why didn't you?

Think of it like this, you have gone through so much that you have almost done the 'zero-mileage' list on your brick. Take a breath, and complete the work as time/money permits.

Greg Mustang
Montreal - Ottawa
Canada