Mouse stutter in Modern Warfare 2

Hello,

I've reinstalled Modern Warfare 2 on my computer but now I'm experiencing mouse lag or stutter when I'm in game - this doesn't happen in menu screens and occurs in multiplayer and campaign modes. Basically, when I'm moving the mouse, the movement isn't smooth and the sensitivity is inconsistent making it very difficult to aim properly and quickly. I've tried fiddling around with v-sync and turning down graphical settings but it's all the same. I've installed Logitech Setpoint but that made no difference. Lowering the sensitivity lessens the effect, but the problem is still there. I didn't experience this problem when I first installed the game but now it's everywhere. Any help would be much appreciated.

Also, on a side note, at the same time my sensitivity settings in Counter Strike Source had decreased for some reason but this was easily dealt with by raising it in the config files. I have no idea what's going on.