games randomly go sluggish during play?

I've been having this issue for a couple months now that doesn't seem to go away regardless of changing drivers etc and appears in every reasonably modern game I've played. Basically it will be going along perfectly smoothly, and then suddenly BAM its really slow and stuttery for 2-3 minutes, and then it goes back to being perfectly smooth - and this just repeats over and over.

My computer is an HP NX9420 laptop, its got a Core Duo 1.8Ghz processor, an ATI x1600 Mobility graphics card and 2GB RAM, and it's running XP.

Has anybody else had a similar problem and managed to find a solution? It's really aggrevating to try and play through an FPS when you've got to deal with it going haywire every couple minutes.

Thanks!