I've been mining for about two weeks now and I just got a new GPU, 6950, to replace my older 5770. Now everytime I leave the computer to mine and go to sleep or to work, it crashes quite soon. 5770 worked just fine. I reckon it has something to do with the powersaving things or the screensaving policy as it seems that when I'm using the computer on desktop (internet, music and such), it mines just fine. I just set the screensaver to "never" but that would just be a temporary solution if it worked. I use guiminer for mining.