Stop all 3rd party startup items in msconfig.
Did the problem go away? If yes, great. Leave those startup items disabled. Odds are that you don't need them anyway.
If not, the issue is either a bad driver, or insufficient system resources.
Resources:
With random jerking, the issue is usually not enough RAM, or a slow disk system if the game is constantly streaming the content from the disk. Open task manager, go to performace tab, let it run on the desktop, and open your game. Play for a bit, ALT TAB, look at the memory usage history graph in the task manager. Are you seeing the usage close to the max amout of RAM your system has? If yes, get more RAM! Also check to see if there's disk activity during gameplay. What kid of disk do you have? If IDE, it might be stuck in PIO mode.
Drivers:
If this is a bad driver issue, try disabling all non essential devices. Unplug all USB devices. (Use a PS/2 keyboard and mouse.) Open device manager and disable the modem, network card, sound card, bluetooth, basically the hardware which uses 3rd party drivers and isn't essential to the game running. Did the issue go away? Disable devices one at a time until you find the culprit; then seek out updated drivers for that device. If the issue is still present, then the problem is most likely your video drivers. This is pretty hard to test for and prove, unless you work at ATI or NVidia. So just update the video drivers to the latest and see if the issue goes away. If not, try older drivers, or borrow a different video card from a friend just to test.