Power Supply or Video Card?
Machine restarts or shuts down (usually shuts down) whenever anything graphics intensive starts. My machine will be on hours and is quite fine until I start running a game and it may still be fine until something graphics intensive occurs and it just goes. My specs are in my sig if anyone wants to know to query I have a generic PSU but its been running the same card fine for hhhmm perhaps almost a year now give or take.
Intel i5 2300, Gigabyte Z68XP-UD3, 12GB DDR3 @1600 MHZ, XFX 6870 2gb (Stock). All Stock
For in much wisdom is much grief, and increase of knowledge is increase of sorrow | If you love something let it go, if it comes back to you its yours if it doesn't, it never was.