game jitter and pausing with ATI card

[torpid]

When playing Nexuiz, UT2004, and Quake 3 Arena at 640x480, I experience constant jittering (acts like low fps) and after several minutes of game play there will be a 1 or 2 second game freeze (but doesnt lock up) then the game resumes.

I have an ati 9600 pro with a 3.4 GHz P4 with 512 MB RAM running Gentoo-sources 2.6.15-r1. I'm using the ATI closed source driver version 8.21.7-r1. I have tried version 8.23.7 and had the same problems.

I have gone through the gentoo ati documentation on site and in the wiki, but none of the suggestions seem to have any effect.

Any help would be appreciated.

~andrew