I can second both of Marks experiences, but I'll also add heat related issues.
Sometimes fans just slow down before they completely stop.
I have a friend who was a tech/help desk person for a fortune 500 company.
Says the first thing he does with every PC problem is reseat the ram. 

At 03:39 AM 2/20/98 +0000, you wrote:
>   I worked on a machine a while back that would shut down and
>reboot, turned out it was a loose strip of RAM. Also worked on
>another that was notorious for shutting down(mostly) when using
>netscape but on closer inspection it also did it occasionally with
>other offline programs. When I looked inside the RAM had been mixed,
>some was parity and some was not. After replacing it with all
>nonparity the problem stopped. Don't know if it pertains...just
>thought I would put my 2 cents in:)
>                                           Mark Walter
>
>> > Hi:
>> >
>> > Periodically my computer shuts off & reboots.
>> > No set pattern when this happens,  just when I'm on the net.
>>
>
>
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