I'm sure you know this already, but if it were me, I would disconnect everything, drives, ribbon and power cables, mouse, k/b, remove any slotted devices, USB connections, everything, leaving only
chip & fan
Try that. If it improves, add devices one at a time--one of them, or the connection is the culprit
if bare bones vanilla mobo still has the same problem, remove ram, gently clean gold contacts with a clean pencil eraser.
if that does not help, I would remove the motherboard and place on paper surface close to the case, make bare bones connections so the mobo is totally isolated--might be a washer/screw short.
sorry if you have already done this, but it was not noted