Paste loses thermal conductivity over a long time (it seems to get kind of dry and brittle). So replacing it every so often is necessary to maintain the lowest temps. Arctic Cooling MX-4 claims 8 year durability which is a lot longer than the typical 4-year or 5-year paste. Whether it actually lasts 8 years, I really don't know (I'm only a year in after replacing the AS5 with it on my i7-920). But yeah, most pastes need a few days to get happy, and then temps will stabilize. Stock heatsink on a 980x? Ick. But whatever works for you. Those temps are fine for the stock cooler on idle. How is it under load?