I don't see how Tremulous can cause anything to overheat unless you've somehow removed a heatsink or whatever. I've played this game for a long time on a machine with nothing but heatsinks, no fans, not even on the CPU. It was damn slow but it worked.
So yeah, in addition to cleaning the dust out of your machine, make sure everything's held down properly. And make sure you're not mistaking the effects of another problem for overheating.