Need help w/fuel gauge

["Hanson, Todd" <thanson@frost.com>]

     I am having a minor problem with my T-3 fuel gauge. When the tank is 
     over half full of gas, the fuel gauge displays the normal reading. 
     However, when the tank gets down to about half full, the needle jumps 
     back and forth a lot. I tried replacing the fuel sender with one out 
     of another car (same year) with no change. I assume it is the fuel 
     gauge or the cable. How do I test these? Thanks in advance for your 
     help.
     
     thanson@frost.com