Comment 8 for bug 1087582

Looks like the processor c-state issue we have on some other 945 machines. If you boot with processor.max_cstate=1 does the problem go away?

The issue is that we rely on vblank interrupts arriving at the correct frequency, and on some platforms when the CPU is in a deep sleep state, it won't wake up when a vblank interrupt arrives, but it will wake up when other device interrupts arrive. That's why you see the performance increase when you move the mouse.

I still don't know the root cause, but if the above works for you, then it's a duplicate of a known bug at least.