Maybe check the wiring at the upper right flipper and opto pair. Could be a wire is pinched or stripped and is touching the flipper lugs or something similar. Follow the wiring from the optos to make sure there isn't any damage. Could it also be one of the optos is loose and vibration is causing intermittent connections? Also check the 16-opto switch board under the playfield. The .156" header can get cracked solder joints and the IDC connector can be troublesome.

I once had an issue with a game where a screw holding down a ball rail was 1/16" too long. It went through the playfield and pierced a GI wire underneath. The GI voltage went through the screw, to the ball rail, to a spinner bracket, down the spinner to the switch arm and to the switch matrix causing (of all things) displays to not work. Could be something as unrelated as that. Just a thought.