Check your temps. Or maybe problematic video drivers. Try running in safe mode to check if it still happens. If it does maybe you have some hardware problem.
Video driver problems could be very possible. My temps always seem to be stable and my cpu is cooled by a Corsair H50 liquid cooler. Speedfan does not seem to correctly read the core temps, but I've never had a problem come up before telling me to shut down.
I'll try running in safe mode soon.