I have a very mild Win7 hobby rig running latest version guiminer with a non-reference 6950 2GB in my x16 slot and a 5830 in a x1 slot (via premade x1 to x16 riser cable). I do not overclock my 6950 since it has never seem to run stable over 820Mhz which doesn't seem worth the overclock. I do overclock my 5830 to 950mhz, and underclock the memory to 450 (will always lock up as soon as I go under 400) It's in a case but with a ton of fans (140mm side mount blows right on cards), and temps seem acceptable to me, 74-76 degrees peak temps on both cards, with about 55-65% fans using auto speed). I have a newly installed 850w Kingwin P/S that replaced a 500W and the 350W that I was using to power the 5830. System seemed to be stable, going 7days + without a hiccup, but now seems to lock up (screen frozen, or blacked out when I turn the monitor on) at least once a day (of course, usually just after going to bed, or leaving for work).

Looking for some ideas on what might be wrong. Dirty power? Should I consider a nice UPS that can output 800w+ ? I'm going to look for some heat pockets maybe in the HDD area, or around the MB/CPU, but I have dual top mount exhaust fans, and 4 exhaust fans in that area so I'm doubting that. Flaky core system components, maybe? I have been thinking about a MB/CPU/RAM upgrade so might just replace that anyway. Just frustrating to have a system that ran so well becoming such a PITA.