Mysterious fuel gauge level indication…..

On the ’73 car the fuel gauge had been stuck on 1/3. I thought it was a bad sending unit so I replaced with a new one. Same problem. I then took both the gauge and the sending unit to Palo Alto Speedo. They said it was now working fine.

Before I re-installed the gauge and the sending unit, I connected the sending unit and watched the gauge move as I tilted it back and forth. OK it works.

After installing I began to notice the gauge was once again stuck at 1/3. Does this ring a bell with anyone? Has anyone experienced this? I thought maybe a bad ground or connector, but all seems fine.

What should I go after/look at?