Could be that you have the a virus , for example fake antivirus spysheriff infecting your PC, that causes Windows to shutdown randomly.
Your PC could be overheating. Check that your fans are free of dust, that includes CPU, PSU and also any fans on motherboard devices.
Your PSU could be under par for the task. I didn't take the time to check your signature, sorry. If a PSU is being tested muchos then it might get hot and have a little lie down.
Perhaps it's just because Microsoft are poo.