I'm pretty sure the ball popper's do not have diodes. How do the ribbon cables look? Do they look old and ratty? I wouldn't be surprised if one of the ribbon cables is bad. Try re-seating them.

Also, run the solenoid test and see what happens to those particular coils.

It could just be the opto's or one of the opto's is really dirty. That's the easy fix some hopefully thats it. Take them apart and clean them real good. Windex works well.

Hmmmm. Well I can tell you I had the same sort of issue with my CBW, 8 ball coil kept getting stuck once it was activated and it would fry if I didn't shut the machine down right away. It turned out to be a bad spot target that was shorting out and once I replaced the target, I never had the problem again. Now I know they are two totally different machines and systems but maybe my issue can point you in the right direction.

Glad to see you got it fixed. Such a relief when you finally find it right?!? Good job. Enjoy that game now. It's a great pin.