Robin Hoods Playground

i have fixed a ps3 now three times for yellow ring of death , i strip it down heat the board up put new thurmal compound on but every time it lasts a few days why is that anyone shed any light on this please

because a reflow is very hit and miss and the ps3 only gives ylod,and we have no way of determining what the exact fault is as there are no codes(unlike the xbox360),which does suck