PC Freezes, then automatically restarts (no BSOD)

[Haruspex]

Hi, i have this problem for a while now. At total random times the pc will just freeze (the sound will freeze as well, if there is any) and after a second it will automatically restart with no BSOD (i have automatically restart disabled in control panel). It can happen in games, browsing or just idle. I think it happens more often in games but i am not sure, as i said it's totally random and actually kind of rare, so it's kind of hard to replicate it.

I recently upgraded my cpu, mobo and ram, so it shouldn't be any of these three, because i have the exact same problem with both the new and the old components.

PC specs

cpu q9400 -> upgraded to i5 4570

mobo gigabyte ep43 s3l -> upgraded to asus z97a

ram 4gb ddr2 corsair -> upgraded to kingston hyperx fury ddr3 1866 4gb

soundcard xfi xtreme gamer

gpu palit gtx 660 (no oc)

intel ssd 330 120 gb

western digital 320 gb hdd

psu xfx pro core 550w

os windows 8.1 update 1 with all the latest windows updates

I have tried running prime95, memtest86+, intel burn test, valley, 3dmark11, furmark, occt and i can't replicate the freeze, it only happens at random times (and rarily). Also i don't see any problems with my temperatures as far as i can tell.

[Evantaur]

i have similar problems with my rig... doing full shut down (power off from psu for 20 seconds, then power it back up) once a week and it's stable as rock... i think my PSU is starting to itself