I have a GTX 580 MATRIX as well and GPU Tweak stopped working 3 months ago when NVIDIA released the driver 301.42. That was when I switched to EVGA Precision X and there were no problems so far. Today I gave GPU Tweak another try, since it's still the only application that lets you control the custom fans. It looked like everything was fine, the guys from ASUS finally managed to give us an option to disable the hotkeys. But that's about it in terms of improvements, since whenever I click 'Apply' or 'Default', the application crashes. This is reproducible with the drivers 301.42 and 304.79 under Windows 7.
ASUS still hasn't learned that superior hardware means nothing when the software is useless.
I dont have a 6850 but I tested it with 6970 I cannot duplicate your error. You should try to remove the GPU Tweak, remove all ATI driver completely out from your system using control panel. Next go to system properties to select uninstall (check on remove all driver from system). Reboot, and start from scratch to install the driver and follow by the GPU Tweak. Let see it will solove your ptoblems.
Change is the law of life. And those who look only to the past or present are certain to miss the future.