Check CPU temps then, and while you're at it GPU temps and any other sensors you can find. AIDA64 Extreme Edition is good for this. Watch the temps while you're mining. The only thing that should be causing the computer to shutdown or restart on its own are heat and power issues... or if you've installed Windows updates and weren't paying attention.
I've been running MSI afterburn with the fans at 75% and the temps are only like 75 to low 80's...so i know its not the gpu temps causing it