I built a custom for gaming a few months back and I'm having an awful time with random shutdowns! When i first assembled the PC, it was less problematic but i still had the occasional shutdown from out of nowhere, instant power off. Here are my system specs
Go into your bios and check the "PC Health Status" there is a shutdown temperature at the top..set it to 70C.
Also monitor the temps for about 10 minutes...if the cpu climbs above 50C then you might have a seating problem...are you using arctic silver5..that seems to be the best out there right now..and you can't cut corners with heat.
Hope this helps.