Stalker: Clear Sky chugging in places

I just installed Stalker CS and updated to the latest retail patch, 1.5.07, I think, and when I would turn, for example, the game would kind of stutter and skip a few frames in the turning process, and the display would take some time and 'chug' to try to catch up with the mouse movement. But after I have turned around and given the game a few seconds, there would be no chugging if I repeated the movement. For instance, if I turned towards point A from point B, the screen would stutter and chug towards point A, but if I waited and turned to point B and back to point A, there'd be no more chugging or stuttering, but a smooth framerate. I am playing at 1650.1080 with High settings and enhanced dynamic lighting (NO DX-10) and managing 50 FPS, so the game runs really smoothly, it's just it will chug randomly but like I said, the chugging will disappear, so I'm thinking it may be a loading issue or memory leak? Has anyone else experienced this? My specs are below:

AMD Phenom X4 9950 2.6 GHz
PC Power and Cooling 610 W Continuous PSU
4 GB DDR2 RAM at 800 MHz
ATI Radeon 4850 at 512 MB
Vista Ultimate 64-bit w/ SP1