Certainly, when a rig develops as issue of just instantly shutting off, a PSU is often a prime potential culprit...
But, given the other facts give, (i.e, none), you are not likely to get much more than a 'maybe' on assistance.
Have you monitored 'under load' CPU temps/clock speeds, to rule out potential overheating issues? (Some motherboards maybe have BIOS settings to induce hard power off at certain temp conditions..)