Please Help: Game characters pause randomly while walking/running

When I try to play games such as Bioshock, Call of Duty 4, and Tomb Raider Anniversary at maximum settings (@ 1280 x 1024 resolution), the games run perfectly with a healthy FPS. However, instead of walking smoothly at a constant speed, the character that I'm controlling randomly stops walking suddenly for around half a second at a time before carrying on walking again. This happens at random intervals averaging around every 5 seconds.

There is nothing wrong with the keyboard, as I've tried various keyboards and joypads with the same result.

Anybody know what the problem could be? I doubt that it is an effect of playing at such high detail levels, as apart from this slight, but terribly annoying niggle, the games play perfectly.

My setup is as follows:

Asus P5B Deluxe
Intel Core 2 Duo E6600 @ Stock
2GB Geil DDR2-800 Dual Channel RAM
ATI Radeon X1950XT
Samsung SpinPoint 500GB Hard Drive

Dual-core hotfix, shut off anti-virus maybe, make sure your keyboard/mouse IRQs aren't interfering with other parts. This isn't usually a problem but you never know.