This might not be causing the problem but what power supply are you using? I looked up the watt usage for a 6950 and its 312 at full stress which x5 is 1,560 watts and that doesn't include the mobo hd ect.
Silverstone Strider Gold 1200W. I have plenty of headroom. Also, the miners crash when I try to start them when there is 0 load.
I'm kinda confused how do you plan to run 5 6950's off a 1200 w power supply if when they are mining each card runs at 312 watts? I may have that number wrong but I looked it up here:http://www.guru3d.com/article/radeon-6950-6970-review/11
Does it crash the moment you start mining with one card or when your mining with 4 cards and you begin mining the 5th?
You do have that number wrong. 312 watts would be for that entire system.
I believe a 6950 requires 2x 6 pin slots. Each 6 pins pulls 75 watts max. The motherboard may also pull another 15 watts per card. That's 165 watts PER card times 5 cards. 825 watts, say a 125 watt CPU and another 100 watts for other stuff (multiple hdds/whatever), still under 1200 limits.
The problem does seem to be windows, have you tried in linux? I think LinuxCoin would be helpful in this situation (live, already ready for a test).