Need Help with fuel guage.. :(

OK...a 1976 PA 28 151..

After a few minutes of running, the port guage will start to fall
towards E.

It is showing 3/4 now (fuel at tabs), and will sag to 1/4 within
about 15 min, while I am operating on the starboard (other) tank.

Shut down for a few hours, guage resumes normal (correct) indication.

We have checked / replaced, tank grounds, sender, wiring to guage.

I think the guage terminals handle 12 V to the left terminal
(looking at it from under the panel) (starboard terminal) , The other
to the tank sender (port terminal).. (grounding this terminal when
the fault is present will cause a "full" indication).

From what I recall, there is a "ground" connection internal in the
guage. Anybody know how this is grounded?

I suspect a bad connection here somewhere..

Driving me up the wall..of course every time I attack the problem,
meter in hand, it works fine.

Anybody any ideas? Suggestions or test procedures?

Dave