Have you tried a different PSU I know you want to focus but I didn't see it mentioned and it's commonly overlooked. If you are sure it's CPU and or mobo and haven't checked the PSU. Take you proc and PSU to a local PC repair shop hopefully one you already have a relationship with but if not it's time to do so. Typically if you do business with them (buy stuff) they'll check things for free otherwise a small fee would be in order either way all that will be left if they are fine is to RMA the mobo. GL
if you go back one page you see Ive posted that ive tried with a different PSU. But Ty anyway.