There is a wiring or gauge problem then.
Simple test of the circuit is to disconnect the wire, see if the gauge goes past max one direction, then connect that same wire directly to ground. It should go to max the opposite direction.
Yours is not acting properly with the sender disconnected, so the problem is not the sending unit.
All the other gauges work properly? This vintage they generally all share the same power and ground, so other gauges would be affected if it was a cluster power or ground issue.