6950 Flash
In situations like this i always to the same thing (revert everything back to defualt) First I would update your mainboard bios (flash it to the lateest) After flash dont change anything in bios. After flash put in your working card boot windows, after windows boots shut down, put in your r6950 ASWELL and boot windows again, Load up ATI FLash do this
atiflash -unlockrom 1
then atiflash -p -f 1 cayman.bin
Make sure you have to 6950 in the master PCI EXPRESS SLOT, Dont put it in the second one. Also make sure the adapter corresponds to the right card. Had the same problem for some strange reason it wouldnt flash it in the second slot (maybe something to do with the PCIEXx8 i dont know. I did these steps and it worked. The only reason i flashed my mainboard was the make sure all my overclock and settings were set to default, Incase my PCIEX slot was disabled or something. after the flash everything would be factory reset. It wont help with the actuall flash but its 1 less thing that you can rule out. (also new bioses are better anyway).