...but when i turn the cpu clock down a little I stell get 770mhash/sec and the card has been stable for about 2 days now.
currently i am at work, but i can upload a screenshot later if you want to. 5970 can potentially hash over 800.
Seems right, i'm mining on an air cooled 5970@730 MHz and i'm pulling 700-720 MH/s. Using a good liquid cooling I guess you can go stable over 800.
On the other hand, with the 200$ for a good liquid cooling you can get an additional 300+ MH/s GPU.