Yes it does seem to run longer when it first starts up. Today I reinstalled tiger and it seemed to work fine. Then I tried to put leopard on and it froze repeatedly (probably because it was warm). I'll try again tomorrow.
I was installing snow leapard when half way through the optical drive stopped working. My macbook continuted to work fine under the windows partition. I installed a new optical drive and tried to fix the MAC OS. The computer kept randomly freezing within a few minutes of use. It still worked...