Like others suggested is that if your PC just shutdown while playing a game it is either your PSU failing or rather it might not be big enough to run your system. What is the minimum requirement of watts your video card requires? Also you have to add in the rest of the system of how much power it also adds to the final wattage used by the PC in whole. Meaning do you have a seperate sound card, keyboard, cd-rom/blue ray drives running, etc.

Try turning down your video card settings or if its an AMD leave it on stock settings and do not use AMD overdrive or the NVIDIA equivalent of that software.