There is a widget for Dashboard (free from the Apple.com site) that displays the temperatures and Fan RPMs going on inside your box.
That will help you to determine which fan is breathing heavy, in the first place, and which temperature sensor is causing which fan to cycle that way.
Armed with that data, you might have a better and quicker (maybe even cheaper) fix to the hardware problem going on.
Remember last Summer whan CBS2 took a PC, unplugged the harness to the power supply, then took it to 10 computer repair places?
hahahahahah
10 different answers. the most embarassing one came from Best Buy!
Independent photographer, film maker and Producer. In the wonderful UK.