You may have a bad memory block.
http://technet.microsoft.com/en-us/magazine/2008.09.utilityspotlight.aspx
It’s a time bomb and may also show up when you do other memory-intensive activities. Machines used to do a memory test when they started, but nobody wanted to wait, so it’s commonly bypassed. You may be able to go into your BIOS, defeat “Fast Startup,” and just let it test.
Koz