If it is the insulator ring hanging out it hasn't been put back right correctly. And it doesn't look right . Its very easy to put these back upside down.
Fluctuating resistance, it's all I can think of, the 510 pin threads and tightens ok, no cross threading, threading in straight?
Loose bit of wire somewhere....I know you have cleaned it....did you take the post screws out, are there spares, is there a spare 510 pin