Engine high idle?

The FPR should be on the ruel rail, disconnect the vacuum line and look for gas in the line, if present then the FPR diaphram is ruptured and the FPR must be replaced.

Ok guys, time for me to throw some money back into the pot. I'm proud of my mechanical abilities but this one has me stumped. Whoever has the magic answer,

1996 Grand Am GT 3.1 V6 auto.

About 25 seconds after start up, the RPM rapidly bounce between 500 and 1000.
It does not matter if I am in any gear or park or neutral.
It doesn't matter if the engine is cold, or at operating temperature.
It does not matter if the car sat over night, or for 1 minute.

It is not a nice slow revvvvv up, revvvvv down.
It's goes bang, bang, bang, bang about as fast as you can say that comfortably. (Almost as fast as a rev limiter)

After about 30 seconds of this, the idle goes to 900 and stays there. But it normally dies if I don't manually step on the pedal and bring it up to 1000.

If I start the car and immediately put it in gear and start driving, about 25 seconds into it, and doing 30 mph or so, the engine looses power for 1 or 2 seconds and then comes back.
This repeats 3 or 4 times.

When this first happened, I thought to myself, "Oh, that's got to be the IAC"
Wrong.

I jumped the fuel pump relay for 5 minutes before I started the car, to make sure any possible air bubbles had circulated through the system and made their way back to the tank. And did a fuel pressure test with the schrader valve removed. (Just in case the valve was causing a dampening effect)
45 with the engine off.
40 35 depending on intake vacuum.
Gauge was smooth while the engine was freaking out.

I have unplugged the IAC with no change.
I have unplugged the EGR with no change.
I have unplugged the MAF with no change.
I have unplugged the MAP and it wont start.
I have unplugged the TPS with no change.
I have unplugged the evap purge solenoid with no change.

I'm ready to dance around a fire and cut a chickens head off, if voo doo is what it takes to fix it.

I would first check all system grounds. It sounds like the computer could be no good. Also I have seen were the ignition module can backfeed casuing the problem you discribe. These problems can be tough to find. I asume you checked for any vacuum leaks and checked the injectors ohms reading?

Just a thought, as you know the idle is computer controlled. GM had the wisdom to make the pcm memory volital, if you loose battery power the car forgets how to idle. This causes symptom like you describe.
It can be manually reset by starting the engine and holding the rpms at a steady 1500 for a couple of minutes. Just a possibility.

Here's tonight's exciting update.

I unplugged the breather tube from the air intake and was able to keep the engine running by spraying starting fluid in the hole.

Keep those ideas coming...

(thank god this is my own car, I have never told a customer that I couldn't fix their car)

Did you try unplugging the 02 sensors?

This could be the PCM but I would not replace it if they are saying "it may have been damaged"

They should be able to test this and determine if it in fact is the issue or not.

I am attaching the process on how to replace it and relearn the crank sensor.

I also attached the process to replace the idle air control valve as this would be more likely the issue if there are no codes.

If there are codes, please supply those and we can determine next steps. Otherwise, I would replace the IAC as it is cheap and controls the idle speed. Let me know what questions you have.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

Thanks