Computer randomly restarts when playing certain games


I'm new to the forum, but a fan of Tek Syndicate so I figured someone here might have a solution. My computer will randomly restart whenever I play Sims 3, Sims 4, or Civilization Beyond Earth. It doesn't seem to be overheating because I only have that issue with these games and I've played much more demanding games. It's also random so it doesn't always restart but it's often enough to be really annoying. Also the way it restarts is strange in that it appears to lose power. My monitors just go black before it boots again. No blue screen or anything. I tried a suggestion to run my games windowed and it seemed to work but it eventually started doing it again. Any suggestions are greatly appreciated. Here are specs 

CPU: i5-4570 with stock cooler

Motherboard: ASUS Z87-C

GPU: MSI R9 270 Gaming TF4 cooler

PSU: Seasonic 620w 80+ Bronze


When restarting, I would first check drivers, then RAM, then PSU.

- of course if you r sure that overheating is not an issue