i have 3 6950s unlokced and i run them at 1.2v and i can get to 930 core and if i go to 1.25 i can get to 960 to pull about 410mhs per card. and 900 is still a lot for stock vcore the 6970 runs at 880 stock core at 1.17v so up you v to at least 1.175- 1.2 range (if you temps are lower then 80c) and then try for 900