random restarts

Hello all, PC has been working fine since it was built in Jan but has recently started randomly BSOD / freezing during or shortly after start up. If I shut down normally then turn it on again, everything is fine even after a 3-4 days of no use. If I shut it down, then turn it off at the PSU...

I've got a persistant problem, it has continued through reinstalls and hardware tests. The problem is this, my PC at random times just restarts. No warnings and nothing in the event viewer, I could be just watching youtube/netflix it restarts. I've monitored temps and they never go higher than...