The fix, I place a thin alloy shim between the existing heatsink & chip, with a good quality thermal paste, I always use artic silver 5(never lets you down)
This is a complete top half strip down to fix (takes me less than 30 minutes)
this is the culprit, as you can see the shim can easily overheat & go concave lifting off the chip.
cut a thin piece of alloy to the same size as the existing shim, i used an old alloy door push plate(easy to cut) & is the ideal thickness
Thoroughly clean the chip with pure alcohol, then apply an even spread of good quality thermal paste(Arctic silver 5) will never let you down
THen apply more thermal paste to the top of the shim
The new alloy shim is under the existing thin shim. I've done more than 20 of these repairs & had 100% success rate I always do the same to the CPU (might as well your already in there).,
This is the CPU with the new alloy shim fitted.