sorry man the grounding is common throughout all of the boards via the huge metal sheet backplane in the backbox. Thats why important all board screws are tight as that grounds all of the boards. J103 is unrelated - study the diagram I sent you for the flipper circuitry.The gnd is supplied by the power driver board.
I suggest you check grounding.
Anyway if you see my last post I have now proven it is mechanical and nothing to do with the electronics. Thanks for all the suggestions
Cheers
Kev