I have a problem with my own non-refernce 6950 that has it only go to 500 MHz under 3D load. It sounds like you are having a similar problem. I am currently working with the card maker going through basic troubleshooting for this issue, and perhaps following the same steps will benefit you as well.
First, please do not use and third party tools to attempt clocking the card just yet.
IF you have the time, and an extra harddrive, I suggst you do a fresh OS install, and try installing using the Catalyst 11.12 driver. To monitor temperatures and clockspeed, you can use GPU-Z. At the very bottom of the monitor tab in GPU-Z is a check-box to have it update while in the backgroud, make sure this is checked, and load up 3DMark 11 or something and let it run.
When it exits, you can right-click the clock and temperature listings, and select the option to show the highest reading. If the clock does not scale past 750 MHz, your problem is like mine, and I suggest you contact the card maker to get some support on this. If it works, we can come to the conclusion that your problem is driver-related, and then deal with that. You've already eliminated temps, it seems, so let's look elsewhere for the problem.
The fresh OS install will help ensure it's not issues like a virus or other conflicting software. I think you must take this step now.