Arctic Silver 5, and similar, silver-based greases or pastes, decline in effectiveness over time once they are used. You may not notice it too much, but they do.
AS 5 is good stuff, to be sure. And also, I hate to plug another product. But for the money, I really think you and everyone else should just dump AS 5 and move to a nano-diamond-based thermal compound with a high percentage particle loading.
JetArt's CK-4800 is only 10% synthetic micronized diamond, and therefore only equal in effectiveness to AS-5. But it's . . . . OK . . . .
Even so, I recommend this:
IC Diamond @ $4.99 per tube, 95% particle loading -- from Innovation Cooling
In the "Cases and Cooling" forum, I did some fairly controlled and rigorous testing with IC Diamond and my own concoction:
CK-4800 as a "base" with micronized synthetic diamond powder from Penn-Scientific (< 2 micron AVERAGE particle-size)
Both formulations gave the same result: a 2 to 3C degree reduction in load temperatures at the same room-ambient for an E6600 processor over-clocked to about 3.3 Ghz -- as opposed to Arctic Silver 5. I took averages of CoreTemp load values in 8-second-intervals over several hours, with AS5, IC Diamond, and my "custom" mixture -- at controlled room-ambient. I do not know the precise thermal design power (TDP) for that over-clock setting, but it was a controlled constant, as was the room-ambient.
Diamond-based thermal compounds do not degrade over time. Goes without saying that diamond has a much lower thermal resistance and higher thermal conductivity -- a stunning difference -- over silver. And -- believe it or not -- the residue can be re-used. With the IC Diamond, which has an oil base that slightly evaporates, you can add a touch of CK-4800 after scraping the residue off the heatsink base and processor cap for re-application.
The diamond pastes are completely inert, with no electrical conductivity and no capacitance. You can make a messy application with no risk of residue damaging your components.
The tube of IC Diamond is good for about three to four applications. Three applications if you include your VGA GPU.