A year or so ago I inherited a Dell E6400 laptop at work. It's a decent machine. I run 2 external monitors off it in a docking station while at work. When my office gets warm, it often throttled down and became a turtle - driving the external monitors is too much for the video chip while it's also working hard on other things.
I blew out the dust and got a cooling pad thingy for it (CoolerMaster NotePal U3 - http://www.amazon.co...oduct/B003ZMF27G/
) and haven't had the problem recur (will have to see how it does this summer before declaring the problem 'fixed'.)
IIRC, Ashton's iMac had some problematic video issues for a while. More cooling may help quite a bit, but cooling an iMac probably isn't easy.