There are no guarantees with BIOS flashes, and many vendors tried to take steps to curb this practice after it became wildly popular. If you have continued problems, I'd recommend flashing back to your original BIOS and running it as a 6950.
This has been known to cause memory issues which could be what you are experiencing. Flashing Over clocking etc are never guaranteed. Revert to the old Bios and use the card as a 6950.