Posted 18 April 2007 - 07:12 PM
One possibility is that the computer is over-heating. Check to make sure all the fans are working, PSU, CPU, case fans, and possibly the chipset fan. If there is large amounts of dust/animal hair/cobwebs/etc, they might not be cooling effeciently enough.
Also take a look through the Event Viewer (eventvwr.msc in the run dialog) for any errors, specifically in the 'system' category.