It sounds like an overheating issue. Not all PC's are made for gaming and will suffer heat issues when playing games. Even with all new parts a stock heatsink will not cool down an i7 enough when playing games.

I recommend monitoring your CPU, and GPU temps with something like Speed Fan. Your computer will normaly shut itself down when CPU temps go above 85c.. But really anything above 70c should be treated as a problem that needs fixing.