Try running stress tests for a few hours (for CPU, for video card, memory, running in a loop), try running in Dx7, try multiplayer, custom battle.
I recommend keeping an eye on temperatures.
Power off could also happen because of:
- video card failure
- HDD failure
- PSU failure
- other component failure
- something overheating
Stress tests should help you figure out if it overheats or not. Not sure how to test PSU, hdd or some other components.
When you uninstalled/reinstalled, did you remove remaining files?
Hard to tell what could be causing such behavior...