I would just check all the obvious:
-Take off HSF and redo lube, even if you're sure you did it 100% before.
-Clear wires out of the way (cable management) if you can.
-Clean out dust ofcourse
-Do you need to put new fans?
-And definately make sure HSf is sitting properly.
edit: And to be honest I never do that pea dot of heat transfer compound in the middle. I do a thin layer all over then some on the surface of the HSF base, with some remove (to fill the micro-cracks). I know potato, potatoe....