Al said:
I can go for a few days without this happening. The screensaver/ power
options I understand and have checked.
Your problem could be totally unrelated to the Operating System upgrade,
especially since your problem can take days to show up, and randomly at
that.
A UPS with a dying or dead battery can cause random power downs.
Same can be said about other stuff like a bad outlet or just unstable power
such as when a hair dryer or vacuum cleaner is plugged into the same breaker
elsewhere in the house. Then again it would not be the first time a
computer problem has been blamed for a loose power plug where it attaches to
the power supply or other actual power problems.
Try plugging in an electronic clock that resets when it's unplugged, as in
no battery backup, into the outlet along with the computer and see if that
too shows a problem when the computer shuts down.