Places where temperature change can effect all the vehicles electrical... bulk head connections, ground strap block to firewall, and batterys' ground cable.
Places where temperature change can effect the fuel gauge alone are inside that gauge or its contact/mounting posts. The little ground jumper at the sender completes the ground path in the steel fuel line only. That fuel line is held to the frame with several little clips and at one time one or all of them were contributing to the ground path. Today there may be enough corrosion to interupt all but one and its intermittent.
Simple check is fuel line near fuel pump inlet ohms resistance to battery ground.
Temperature change is what makes the needle move. Heat from the nichrome winding tranfers to the studs and the nuts.
Clean any rust off those studs. Replace the stamped tin nuts with internal toothed washers and 10-32 hex nuts identical to those on your ALT gauge.
