Second time I've come across this prea ground connected to casing and separate pa ground connected to casing via standoffs to metalwork. Underside of pcb large solder pad, upperside 2 pieces of tinned copper wire. The underside solder seems to compress , maybe pcb as well, and then exceed the give in the star washer and looses good grounding. This time prea end , previous time was pa end problem. This time will solder brass washer on underside,each end, solder just between washer and board , not standoff touching surface.