Does it have opto switches under the flipper buttons on the cabinet?

Clean the inside of the U-shaped optos thoroughly, and then throw the game into switch edge test to make sure the flipper switches show up steadily and clearly.

If the problem persists after cleaning the switch optos, swap the little boards left-right and see if the problem moves to the other side.

RM

Full flipper rebuild kits are generally a must-do upon any new acquisition.

RM

When you get an intermittent flipper opto switch, sometimes it's the interruptor, and not the opto itself.

RM

That issue, and I have seen where the interruptor was only barely moved out of the way when the button was depressed all the way, I mean right at the edge where the light was only partially unblocked and it would "chatter" the opto switch open/closed.

RM