The Problem is that SOME MACBOOKS are comming off the production line with TOO MUCH thermal grease and as a result the Heat is being dissapated everywhere into the machine instead of just INTO THE HEATSINK.
THats why when you clean off the old stuff and put on your own in a more moderate fasion... the Heat gets Dissapated off the Processor into the Heatsink Directly, which then gets cooled off by the Fans the way it should.
It seems that it should be an easy fix... just lower the amount of thermal grease that gest applied to the heatsink.
It should be corrected in the next revisions.