I will say this...
I used to have that problem... a lot of times when just Firefox was open. I recently went out and bought a 320 gig drive to replace the 160gig. Cloned it before installing and now I don't nearly have that problem anymore.
I'm cheating...
I have a metal plate that has 2 fans underneath that I put the laptop on. USB powered. Stays pretty cool that way. Might have to try a test without the cooling plate.