Now that is rude.
Rate of heat dissipation is proportional to temperature gradient. So the rate at which heat moves from heatsink to atmosphere is low when just starting, as the heatsink is cold. Therefore the chip gets hotter fast. Once the heatsink starts to warm up, it throws out heat faster. After a while the chip and heatsink hit equilibrium, at which point you hope the chip is under 70. It's often called Newtons law of cooling, and it's a poor approximation but it is easily accurate enough here.
Beyond that, form your own notion of stabilty and work to that. ******** to you.
My last post was not aimed at you. It was aimed at the pos