All ground wires must be continuous and intact throughout the entire wiring harness. The frame ground wires need to be checked: monitor, coin door, BOTH control panels, and speaker grills. Be sure the power cord has not had the ground pround snipped off/removed and that the machine is plugged into a properly ground power outlet.
The gameboard edge connector foil contacts could be tarnished and need to be cleaned. If your machine has that troublesome "Static Filter Board" plugged into the game board then try temporarily bypassing it (unplug the harness connector from the Static Filter Board and plug directly onto the gameboard instead).