Stuttering in games

I'm having a problem with lag/stuttering in games that are easily getting 60fps. When I'm playing bad company 2 sometimes if I try to use the mouse to turn it won't do anything and then it will turn but stutter and kind of frame tear. Another example is call of duty 4- Very often the mouse becomes a bit unresponsive and the game stutters a lot.

My specs are

Core 2 quad 2.66ghz - Same problem if I have it overclocked to 3.2ghz or if I have it stock. Zalan cnps9700 cooler

4gb ram

Radeon 5870 - Upgraded yesterday. Same problem when I had my gtx 280 yesterday

Gigabyte EP43T-UD3l Motherboard

Cooler Master GX-750 PSU

Please help me if you have any ideas!!

Thanks for looking