Paul__B:
No,it wasn't "parasitic" capacitance it was the "Y" capacitors on the interference suppression module.
Good point, likely correct.
Watcher:
Did you ground the metal enclosure of the arcade game and power supply box or the low voltage side of the power supply?All metal enclosures shouldn't have been grounded according to local regulations anyway?
The metal chassis inside the game is connected to the ground on the AC cord. This includes the metal doors for the coin acceptors and coin box. And probably the metal shafts on the joysticks.