In my experience (18 months vaping, 12 months rebuilding), coil resistance should stay pretty much solid if, as Simian said, all connections are tight. While coil resistance does change as the coil ages and repeated heatings take their toll, the usual effect is for the resistance to go up, not down. Even then, I wouldn't expect more than a tenth of an Ohm or two over a couple of weeks of vaping ...
I note that the OP is using a Rocket RBA, which—I believe—is a Kayfun Lite / Russian 91% clone? If so, I would suspect an intermittent short between the deck and the positive post. In many of the clones, the thin insulator between the positive post and the negative deck can wear and slip out of position. I have both the Kayfun Lite+ and the Russian 91%, and their insulators appear to be much more substantial than the clones I've seen. Another possibility is the coil sitting too low and making contact with the deck, effectively taking a wrap or two off the coil, thus lowering the resistance.
Or, it could just be gremlins, elves, and evil faeries ...
