Problems with GPU miners

[FotG]

So far I have tried GUI miner, and phoenix with phatk kernel, I also tried some java based one but forgot what its called. I always run into a problem where I get either a white screen or a white screen with columns (http://i55.tinypic.com/24vqiw1.png) and the system freezes. This however does not happen when I am mining on only one card. I have a 5850 and a 5830. I have tried both with crossfire enabled and disabled. The crashes are a little worse with crossfire disabled (sound stops playing). Neither of the cards are overclocked. And I can use both of them with folding at home for weeks on end with no problem. Temperatures stay below 82C on the 5850 and stay around 70C on the 5830. What could be going wrong?

My system: 5850
                5830
                stream 2.4
                video driver 11.6
                Windows 7 ultimate x64 sp1
                850 watt psu

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[noyvpom]

I was getting similar problems with one card. It turned out it was the PSU, although your PSU 800W should be plenty. Maybe check if your PUS has multiple 12v lines your not using all the same one?

Try cgminer, I have found it to be the one I like most. you can set it up to automatically  adjust core speeds / fan speeds to stop it going over temperature etc.