Unfortunately, there is no good way for us to determine why your computer might be shutting down. Yours is the only report that we have seen of this sort of thing happening, and it's likely caused by a unique situation on your computer, such as an intermittent hardware problem, that it causing it to shut down. It might still be a good idea for you to have it checked out by a competent computer technician.
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Barbara
Unicorn
Last Day of Work