The opto boards were a poor design by Williams in that the resistors tended to get very hot and play havoc with the optos on the board.

You will be able to see this in the switch test in the row of 6 indicator "squares" for the ball trough on the DMD will have at least one that is usually flickering on and off.

In nearly all of the cases the problem lies with the transmitter board only - this is the board closest to you on the ball trough when you lift the playfield to the pulled forward horizontal position.

If you are not up to soldering then you can pick up a new improved opto transmitter board for about $40.

Right at the bottom of the page here:
http://www.littleshopofgames.com/boards.html