Possibility:

If it has that long 7 opto trough it could have divots in the edge of the metal the balls roll down. If they are bad enough, the ball over the popper will get fed to the shooter lane but the rest of the balls stay where they are due to the divots. This flips out the switch matrix and it just starts firing balls at will.

I am assuming this game has a 7 opto trough...and the above chart is tough to read, but...

Unless I'm mistaken opto switch 46 is the 'trough jam' switch. If the game sees this switch OPEN (blocked by a second ball in the chute from trough to shooter lane) the game will automatically start firing balls up the chute to clear the 'jam'.

To see if it's intermittent, enter switch edge test and just let the game sit for a couple of hours. Check on it occasionally (or when you hear a switch transition sound cue). If 46 is displayed as the last switch registered that is your problem.

Reseat the connector on the board for column 4. Inspect the green/yellow wire in pin 4 for separation from the IDC connector pin.

Ignore the loop switch for now....it will not cause zombie multiballs.