I encountered a similar problem in my office that hat me hitting my head against the wall for about the week, thinking it's overheating problem.
It turned out that the computer had automatic windows updateds turned on, and the machine set to automatically shut down and reboot when it was done (no prompting).
It finally gave me a clue when the guy complained that it was happening same time of the day, few days in a row. 
I don't think that's going on with your computer though.

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
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