Random Shutdown during gaming.

Hey guys I just want to get your thoughts on something. I built a pc not to long ago and had no problems with it. Now I play Chivalry Medieval Warfare for a couple of hours and the computer just shuts down. I haven't had that problem with any of my other games yet. I play sc2, d3,CoH,skyrim, etc... This is the only game I've noticed a random shut down.

Specs:
Amd Phenom processer 3.4
16gb of ram
ultra 750 watt atx psu
msi 970a-g46 mob
geforce gtx 660

I don't think its a heat issue because everything is fairly new and I have 3 fans in my case. So it could be my psu or mob is what I'm thinking but I'm not 100% sure. So askin for help here...