I assume you tried setting Distance Rendering to low and enable time slice shadows in the graphics options, enable LockStepMode, etc? Unfortunately, it sounds like you've already gone through most of the things reported to have helped others with this issue.
If you haven't seen them already [EDIT: I think you have; I just saw your post in the Steam forum], check out the topic cant get over 7fps
and the links it contains, and maybe the D2:ED topics below.Choppy game - possible fixFor people with stuttering graphics
I don't know if it might theoretically help, but you could try the 4GB Patch
to increase the amount of virtual memory that the game (or any 32 bit application) can access in 64 bit versions of Windows.