Found these quotes on another forum with the same problem,
"Turns out one of the PSU's motherboard connection cables were loose. Got it plugged in all the way and now everything works perfectly fine."
OR
(this guy has the same mobo as you)
"Managed to finally make a restore from earlier restore point. Had to F8 and go to repair mode.
It seems to shutdown for now. Something went south this month when updating software.
Ok, after some more testing, it seems like the issues is Nvidia 304.48 Beta drivers.
I don't what causes them to conflict with my HW / Software, but they seem to be the issue."
My next step would be to try a system restore, back to before the updates were installed, check if the auto-restarts still persist, but if you are using those Nvidia beta drivers that's a good place to start.