gas gauge problem

jaimus said
.......... good ground on the tank,....maybe the printed circut board on the instrument cluster has a hairline crack in It...

poor ground yes, I would think a PC board crack would be pretty rare on these

RedFish said
My guess is the stamped tin nuts that attach the gauge contact posts to the pronted circuit board are loose.

Definite possibility

Along with the good advice so far, don't discount a bad connection in the sender wire especially the end connector at the tank, which is subject to corrosion and internal breakage.