I was going to suggest the same as I had the same problem on a GM car. The sender is basically a potentiometer and cleaning the wire wind with Q-tips dipped in acetone cleaned up the varnish and solved the problem, but since the OP said he already replaced the sender and the gage, twice, I'm out of ideas too other than checking that all electrical connections are clean and tight.

A high resistance connection adds resistance to the circle and will screw up the gage readings.

Duke