I'd guess you have a bad sender, especially if it's an old original the wire wound resistor is corroded .... take it out and operate it by hand to see how smoothly [or not] it moves the gauge

Bill


Moved over to the Passing Lane

"When we tug a single thing in nature we find it attached to the rest of the world" John Muir
"When we tug a single thing on an old truck, we find it falls off" me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot