So, I finally managed to find time to take apart the PC and reseat the CPU heat sink. Removing this heat sink was very difficult. I had to take out the motherboard from the case, but after that it wasn't that hard. The cooler is pretty large so there isn't enough space inside the case to work with. There is a special Allen key that is required to remove the heat sink. I even had to remove the RAM modules because they were restricting the movement of the Allen key. BTW, if I lose it, I'm screwed. It is pretty hard to remove the cooler with regular tools. But I guess it is not impossible.
I used Arctic Cooling MX-4 for thermal paste. I had a tube of it lying around. That's what I was using before this. It is easy to apply. I used a card to spread the paste sparingly on the IHS of the CPU.
Installing the cooler back was not so hard since the motherboard was outside the case. But the whole process took a lot of time. A whole Saturday afternoon. I also cleaned up the dust in the case and arranged to the cables a bit. The back side of the motherboard tray is a cable mess. Thanks to the design of the case, it is well hidden. Just wish I had a PSU with modular cables. It would be much cleaner then. And the fan controller is not helping either. It has 12 wires coming out of it and the wires are not colored black. Yuk! These Japanese people don't know anything about aesthetics. (The fan controller is made by Scythe and that is a Japanese company.)
The heat sink didn’t need much cleaning though. There weren't much dust on it. Looks like the fan filters of the case are doing a pretty darn good job.
But unfortunately, everything that I did was in vain. I didn't see any improvement in cooling performance after the reseat. It was just like before. I wonder if I should have put a pebble of paste on the IHS and let the pressure of the cooler do the spreading. Or perhaps the stock thermal paste was better than MX-4. Or perhaps this cooler doesn't work well when placed in "pushing air up" orientation. In other chassis' , the cooler would be installed in an orientation where air would be blowing from front to back - horizontally. In my case, it has to be blowing air vertically. Plus, it could be why the fan vibrates when the RPM is between 1200 - 1600. The fan itself is so flimsy. Not rigid enough.
These are the temps when I transcode videos using Handbrake. (Look at the max temps. Realtemp was reset before every test.)
