[camaraderie]

Trantor...sometimes it is simply a matter of poor contact electrically between the zinc and the shaft. Shaft needs to be birght metal...and when you install the zinc you need to tighten the bolts then pound the zinc with a mallet onto the shaft, then tighten the bolts some more.
Pardon the "basics" lesson if you already know this but thought it might be helpful for you and/or others.