Jumping ohms is always caused by a shitty connection somewhere in the circuit. As the same thing happens on different mods it is unlikely that the mod is at fault so it must be something not quite right in the tank.
I've had a difficult time recently with a Tauren One RTA - in this case the build deck pulls out completely and it is the 510 pin that holds the entire deck in place - the negative circuit relies on pressure to make a solid contact and the puny 510 pin isn't really up to the job. I put a tiny sliver of aluminium foil on the bottom of the negative side of the build deck and that sorted the problem.
I'm not sure which version you have - if the build deck negative is a solid part of the bottom of the tank, rather than a separate section held in by a separate screw this won't be the cause, but if the negative side of the build deck is held in place by a separate screw the tiny sliver of kitchen foil may do the trick (just don't put it anywhere near a mech mod after this type of bodge).
There are 2 other things I would be suspicious of - the dodgy grub screw is the main culprit, a potential second is does it have an adjustable 510 pin?. If you haven't done it already COMPLETELY strip it (including removing the grub screws and separating the adjustable 510 pin contact. Remove all the o-rings, glass and 510 - put all the metal parts in a small saucepan of strong washing up liquid solution and boil it. Leave it in the hot water for a good few hours to loosen off any baked on cack then boil it again. Finally wash it off in fresh water, re-assemble (with a new grub screw in the dodgy post) and hope for the best.
