Okay, that system preference isn't available with a MacBook on Leopard. Just thought I'd say.
Anyway, so today I thought that perhaps the problem was that my fans were clogged with dust. I hadn't cleaned them since I got my Macbook over a year ago, so I opened it up (which would have effectively voided my warranty, had it not already expired) and found that my fan vent was completely blocked with dust. And a thick layer of it at that. I cleaned it out, and am running PS now, we'll see if the problem pops up again...