Quoted from kdoggity:
Thanks so much for your help, all.
Unfortunately, I’ve tried everything described here, and am still experiencing the issue. I’ve never taken a trough apart, but it looks somewhat simple to remove the opto boards to check ‘em out... Should that be my next step?
For what it’s worth, it seems like the issue occurs most often when there’s a ball in the lock. Any chance that’s related, or likely total coincidence?
It could, yes! I don't have a matrix chart for this game in front of me, so I can't check, but it's possible you have a bad diode, and it's causing switch matrix issues.
The first thing I would do is checking the trough - the easiest way to check for divots or stuck balls is to play a game with the door open - as soon as you see it happen, and before the ball in play drains, look at the underside of the trough - look at the balls in the trough, and make sure that they are all in line, all rolled down to the front.
You can then put an extra ball in the Lock area, and do more switch tests in the trough - see if anything changes.
You mentioned something about a bad opto, it wouldn't hurt cleaning the boards off when you remove them, too.