so i was right about a bouncing fuel pressure needle/

Yes you were, dose the needle bouncing indicate an issue with the fuel pressure? I guess I assumed it was from the vibration of the motor and it was normal, but that is the first time ive ever had to check fuel pressure so I really don't know if it is normal.

im starting to lean towards replacing the IAC also since everything else seems to be checking out fine. I think im going to remove it, clean it and try it and if there is no change, then replace it. I hate to throw parts at it since I just bought that scanner with the hopes that it would save me money by not having to throw parts at it but I guess it was not a win this time. I could not find any info telling me were the volts should be set at for the TPS in my mercruiser books, only info about install and removal. I did a search and found somebody stating that for a 454 mag mpi gen v it should be between .45 and .65 volts so I set it at .55 volts.

I opened the secondary butterfly valve enough so that when the engine was warm and idling, the IAC was hovering around the #40 position. one time we were out on the boat and going threw a long channel before getting to the ramp, and the dying issue became a major issue while I was trying to dock, I opened the engine compartment and turned the idle up and it seemed to help the issue a lot. when I put the scanner on it (next time we went out), the IAC was hovering around #3 or #5 position. I know that is not correct because it doesn't give the IAC enough "room" to do its job but it helped the issue, that is a "+1" on making me think maybe the IAC is faulty in a way that its not moving fast enough to do its job. of course im just brain storming.

Was hoping you would pull and reinstall the plugs with the motor running. Doing this method you can here the difference before/after and compare.
The timing is moving around, but this can also be to compensate for other issues like the IAC.

Would like to add a correction. Heat and heat soak is way worse in cars. Far, far worse.

I had a bad experience one time while I was trying to shut off a snowmobile that started dieseling, it was late at night but that shock gave me the energy to power on for another couple hours until I go my sled running. I was about 15 mins from loading it up and going home LOL. assuming that you have tried this before and snowmobiles must be different, is it safer to take the plug off at the cap? or should I be taking it off at the spark plug its self?