This is a fresh install of Ubuntu 8.04
Error is reproducible with metacity or compiz.
After about 15 minutes of usage the desktop will freeze. The mouse will keep moving, but nothing will happen. ctrl-alt-backspace doesn't work, I have to do a hard reset.

Possibly related:
sometimes the mouse starts to jump when I move it. It's like it's being swtiched on and off in even intervals.
I don't even know how to troubleshoot this.

Ubuntu 7.10 ran fine on this PC until recent upgrades. Ubuntu 8.04 never worked fine, but this one is a completely new issue.