Have you installed the patch, assuming you don't have an already updated digital distribution?
Try emailing Larian; see the
DKS support page.
I don't recall anyone else reporting this problem, and the only conflict with HDR I remember was getting it and anti-aliasing working at the same time...
Have you tried a lower resolution, disabled crossfire, checked for updated video drivers, etc?
There is a slider in the advanced graphics options to change or disable a frame rate cap (original release is set at 30 FPS). Before I got to your second edit I though it might be worth a shot changing that to see if it made a different; afterwards... it still couldn't hurt to try.
At least with D2:ED a couple different graphics issues could be resolved by running the game as administrator, and in Vista/7 disabling desktop composition in the compatibility tab of the program/shortcut.
