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.