Fuel Guage Goes Up Then Partly Fades -- 72 Swinger

Steven Vames said
Hi all. Apologies for the long story.

I'm on a quest to have a working fuel gauge in my 72 Swinger-- I've read virtually every post on this site and I clearly still have a lot more more troubleshooting to do. But as I go through everything, I'm having a problem that makes little sense to me.

When I turn the key, the gauge moves up to a fair representation of where the fuel level is. But then over the next few seconds, it fades back to roughly half of what is a fair representation. See the video below -- all I did was turn the key.

I've seen similar behavior in temp gauge also but that wouldn't happen until the water is hot/engine up to temp. Switch off then on again and needle would go quickly to max then drop back to proper reading. Solid state regulator installed, all connections cleaned, stopped the wild needle swings.