"I have distortion in an old Vibrolux that only manifests itself on a G note. "
I have a buddy with a BF Showman. It does that same thing, but on F#. In his case, it's the speaker.
"the eyelet fiberboard may be shorting to the chassis at times. They tend to absorb moisture and warp, also conducting parts of the circuit incorrectly."
The eyelet board can also become conductive. I have watched my tech scrub them with a brush and, I think, pot cleaner spray.