Very interesting. That's helpful in showing that the machine is indeed paying out correctly for what it should be nudging to, it's just nudging down each time even when it should be nudging up. In fact at the very start of this latest video there's an incorrect nudge down that should have been up, too.
I would suspect that there's a problem with your MPU board. Either a replacement or repair of your board is likely in order.
Of note, your board is probably fine for use with games that don't nudge, or one of the oddball nudge games like
Knock Down that only nudges down, so don't trash the board if you do go the replacement route -- it's still fairly useful in its current condition.
I guess one thing you could try at this point since you don't have much to lose would be a RAM clear. I really don't think it will fix the issue, but it won't really hurt anything to try, either. If nothing else, it would be cheaper than buying a new board.