Did you by any chance flashed your BIOS with 6970?
EDIT: Check this thread out and see if it will give you some ideas. Also, if you flashed your BIOS with 6970 BIOS, you may need to do VDDC mod.
In all honesty though, these cards can overclock close to 900/1400 on stock voltage.
And my card freezes at 900 and above core speeds.
Edited by Conjuror - 3/4/11 at 6:49am