You MAY want to try this for your dust-filled heatsink. I can understand that dust may be so hard packed that a blast of air does not remove it. Real mechanical force may be needed. By that I mean using a thin tool - knife or small screwdriver or nail file maybe - to reach into the spaces between heatsink fins and push / scrape that dust out. Collect the dust blobs as they are removed so they don't get into other things. Often to achieve this you have to get the fan out of the way for access to the fins. Examine your cooler system closely. On many the fan is fastened to the fins by some screws or springy metal wires and you can remove it WITHOUT removing the entire system from the top of your CPU. When the fins are cleaned out you replace the fan. Whether you want to do this depends on your confidence in doing these careful actions.