You know what's funny? My first temp control mods was the rDNA40 and the SX Mini M Class. With the rDNA40 I was using an Atlantis v2 with the Atlantis nickel coil. Worked fine, resistance never varied. I had a nickel OCC in the SX Mini and the resistance never varied (or not that I had noticed). If anything, as the coil got older and got more gunked, I would notice the resistance climb slightly. The day that I got my VT, put a nickel OCC into a Subtank Mini and started noticing the jumping resistance, especially seeing the coil register as 0.16 and then after a few minutes seeing it drop down to 0.14 or even as low as 0.12, I decided to put a second Subtank Mini with the nickel OCC on my rDNA40. The first time I put it on, the coil registered as 0.19 ohms. After a few minutes, the resistance also dropped as low as 0.12 ohms. Went through 4 different coils and they were all like this. Finally found a coil that first registered at 0.16 and then dropped and leveled off at 0.14 ohms and stayed there. So what I have concluded is that this is just how the Kanger nickel OCC's are and there is nothing wrong with the mod itself.
When I put my Atlantis on there with the nickel coil, it registers at 0.14 ohms and never fluctuates.