i'll try to keep this short
a year ago my rig started shutting down after a few, switching the button on/off would reboot to shut down again after a few.
swapped PSU same issue so instead of screwing around since i had a spare budget mobo, cpu, ram and gpu, i bread boarded that with ssd/HD from my build/case. after a few weeks of swapping everything out, (except the motherboard) out of the case and everything working fine; i feared it was the mobo but decided to go for it still in the case - i was missing the water cooling.
now i should mention that i have a habit of putting "small stuff" in the bottom of my case for safe keeping; screws, cables, flash drives . . . while putting stuff back in i noticed a screw i didn't recall seeing before.
so, everything worked like a charm and figured the screw got behind the mobo whilst laying the case on its side tinkering around and missed the screw.
so maybe just go full anal and breadboard/replace what you can and go from there.
sorry if thats not much help - i keep my screws in a box in my drawer now, btw.