Oh. Okay - on the underside of the PF (lift it up, rest it against backbox), there are two boards mounted down near the bottom (which is the 'rear' of the PF when the playfield is down) of the game, opposite of each other. Those are the two opto boards.
Production games have one board mounted there, and it's a 10-Opto board. Samples have 2 7-Opto boards. They're interchangeable, so unplug both of them, unscrew them, and put the left one in the right position, and vice-versa. Then, see if issue goes away (new issue may appear, but that's fine..) or the issue remains.
This test is isolating the issue - if the issue is on the 7-Opto board, or in game wiring.