Couple ideas here.
I would try to isolate the problem to the driver board or playfield. Here's what i would try: put the game into switch test, then disconnect female connectors 2J2 and 2J3 (the switch matrix row and column connectors). Take an alligator jumper and clip it to the male header 2P3 pin 7 (row 3), then touch the other end of the jumper to 2P2 pin 9 (column 1). The displays should *only* show the number 03, indicating the mpu is seeing switch 3 (the credit switch) is closed.
I'm kinda suspecting that it's going to be a playfield/cabinet problem since it's doing the same thing with both driver boards. Still, that's an easy way to test the switch matrix to isolate the problem to the board or playfield/cabinet wiring.
Given the boards are good, the next thing to check is the wiring to *all* the switches in row 3 and column 1. You want to see if any of the diodes, wiring, or solder lugs are touching anything adjacent. Metal brackets, other switches/lamps, screws etc. It may be like finding a needle in a haystack, but that would be the next thing i would try.
Hopefully that points you in the right direction. If anything maybe the bump back to the top will get others attention who can chime in with more info.
Good luck!