This.

Likely one of the IDC connectors for the ribbon cable has started to break connection intermittently. All those boards (sound, display, fliptronics) are handled on the same cable, and if an address (or even data) line goes bad, or is in the wrong state, an address for an audio bit can get interpreted as a command to fire a flipper. And so on.

Get both new cables - the larger one and the display-only one, just so that you know they're both up to snuff. (You can, also, make your own, which is what I do, but I'm sadistic like that.)

What does your Switch Test matrix look like?

Okay, then. Did you get NEW ribbon cables? (I don't remember if I read that in this thread or another.) I make my own, but many folks don't wanna deal with that.

The next step would be to disconnect (with the power off!) the Fliptronics board. Pull the ribbon cable from it. Turn game on. Does it still act up? If so,
Plug the Fliptronics board back up, and disconnect the sound board. Turn game on. Does it still act up? (Ignore the Sound Error. That's expectd since we disconnected the board.)

If you have not gotten NEW ribbon cables, order some.

Okay, thanks, missed that! D'oh.