I was having the same problem. I think it is the center pin. Condensation builds up in the AFC section where the
rba/Coil connects to center pin. When my resistance starts being wonky I just take a cotton swab or piece of tissue and wipe it. It's good for a while after that.
ETA : I first noticed something was strange after I gave it a good cleaning and the resistance stopped jumping. Made me wonder why that fixed it. Ever since I have just been wiping the center pin connector inside of AFC when resistance is wonky and all has been good.