If you're using a poorly-machined heat sink, using more thermal paste is not the answer. Thermal paste is designed to work when it is compressed to a thin film. If your HS isn't flat or isn't seated completely, as previously commented upon, that is the problem that must be taken care of: lap the bottom of the heat sink until flat if that's what is needed, or remove and reapply the HS(including a cleaning off of and reapplication of the thermal paste you're using). Good luck.