Screws for the coils may be loose. Even if you tighten them, they will occasionally need to be tightened again due to the constant heating/cooling of the metal causing it to expand/contract.
If the 510 pin isn't making a very good contact with the mod it can cause the ohms to jump, can try screwing out the pin on the RDA a bit to see if that helps. Or if the pin on the RDA or mod is dirty.
If the 510 threading on the RDA is loose then ohms can jump as well due to not having a good electrical connection. I have a clone RDA that is like this, the threading is very loose and I can easily wiggle it around if it isn't screwed down all the way. I've had it jump from 0.2 ohms to 0.5 ohms while taking hits. Either have to wiggle it around while taking a hit to keep it connecting good, or really screw the RDA down tight.