| I agree that the grounding of the motherboard is probably not your problem. That said, motherboards have multiple points on the board where they are grounded to the case. There is a solder ring around some of the mounting screw holes. That makes a connection to the case through the metal standoffs or raised bossed in the case.|
You can try using one of the motherboard mounting screws as a path to ground. As stated many of them are meant to ground the board to case. The others are insulated by the PCB of the board.
The power supply may be the source, as suggested by fingers.