Intermittent issues like that are often caused by cracked solder joints at the connectors. Locate the opto board and take a close up picture of the solder joints of all connectors and post it here OR if you are handy with a soldering iron , take pictures , reflow all solder joints on the connectors and try again. You might just solve your issue this quick , but I'm still interested in seeing the pictures. If it isn't the issue, the connectors themselves , their pins , wires going to the connectors all need to be inspected for anything loose that could cause intermittent contacts. There's a lot of vibrations in pinballs , which can easily lead to things working intermittent with their connection isn't solid. If the issue is on the opto board itself it usually costs only a few dollars in parts to repair as most of the time it uses very common parts (LM339 is the main part usually and a few resistors / diodes).