Ok, I've got a really strange problem and I've ran out of ways to try and fix it.
I've got the same "pop" from my MS as the one discussed in this thread:
POP when turning ignition on (not cranking) and burned fuel smell
Now, I've been able to minimize it by holding the accelerator down for a few seconds before I turn the car on.
So a few weeks ago I noticed my mechanical boost gauge was only showing about 15 inches of vacuum at idle. I connected the laptop to the megasquirt which showed 20 inches of vacuum. I then used a vacuum tester gauge which confirmed the numbers from the MS. So I tried moving my vacuum line to the port beside the MS's port but it still showed 15 inches at idle and suddenly it was showing that I was running 10lbs of boost (not possible with my setup).
So a friend was nice enough to loan me his electrical boost gauge but it is showing the same numbers as my mechanical was. Until Saturday night...
That night I forgot to hold down the accelerator and the car "popped" again. When I drove it home the boost gauge was correct. But the next time I did the pedal trick to eliminate the pop the gauge was wrong.
So it seems the "pop" somehow makes my boost gauge read correctly. I understand how the sudden pressure wave could cause the gauge to act funky but I don't get how it fixes it or even modifies it's readings after the event.
If anyone has any clue as to something I could try please let me know. I've kinda ran out of logical explanations for this. It would seem like the mechanical gauge would read pressure compared to ambient regardless of any strange "startup measurement." It also seems weird that this only affects gauges and not the internal map sensor in the MS or the vacuum tester gauge. Although I think the accuracy of the MS could be explained by the length of vacuum hose from the manifold to MS letting some of the pressure wave dissipate.