Thank you
@Darksoft for explaining about the V and where to measure it from. Makes much more sense now.
So I took the V at the Jamma connector and at the solder point directly under the cart slot.
I took them at 3 different times and what you said immediately made sense.
With no cart loaded and at the crosshatch screen both points read 4.758v (while the power supply was still at 5.25v)
Then I measured again with Metal Slug 5 MVS cart playing in attract mode, and both points read 4.646V.
Then I measured them again with the multimvs loaded, and even at the options menu with no games loaded it was already reading 4.514V.
So it made sense to me that once any of the games loaded I started getting sound issues, and other random glitches the longer I let the games run.
I then rebooted into the multis option menu, and changed the voltage to 4.7V at the Jamma edge and cart slot. Still had immediate sound issues upon booting metal slug.
Rebooted, changed to 4.75V, and again still sound issues upon booting into metal slug.
Final stable performance seems to be when Jamma edge connector reads just over 4.8V
I adjusted to 4.804-4.816V.
That seems to keep it above 4.796V. It seems like if the voltage sags too much below 4.8V during gameplay that’s when I start getting sound issues, and random glitches.
Seems like there can be a small difference between the voltage at the Jamma edge connector and the cart connector. Cart connector voltage can be the same or sag to 1/4-almost 1/2V lower than the Jamma side. Does this seem right?
At this point fingers crossed, it seems like everything’s good now

.
Played a few games of each, and have had Last Blade running on demo loop for nearly an hour now.
End result: mv1f board, latest bios and menu, the point on the multimvs cart cut, and the voltage at the connector adjusted to 4.8V and it all seems great now.