We are currently investigating the PC "stuttering" issue, which unfortunately wasn't found during extensive compatibility testing. We believe a few things may be contributing to this:
1) A combination of driver issues and the video memory manager that's used in Vista and Windows 7 under DX10 and 11. We're talking to Nvidia and ATI about this. This may also be responsible for the extreme memory usage that some people are seeing.
2) People using settings that are too high for the amount of VRAM in their video cards. Please note that if you adjust the texture detail settings then you must restart the game for this to take effect. There should be a dialog that advises you of this. Please try using the default Normal setting and seeing if that solves or reduces the problem.
3) The V-Sync and/or Reduce Mouse Lag options. The latter is found under the Controls options menu, not Video. Turning both of these off may help on Nvidia cards. This may be related to the problem described in point 4.xiv in the ReadMe.
As has been noted in this thread, playing the DX9 version instead should also solve this stuttering.
With regards to the low process priority that some people have noticed, this is intentional when you Alt-Tab out of the game, unless you are playing multiplayer. When you Alt-Tab back in, the process priority should return to normal.