[hatzie]

The gauge is going out. Specifically the calibration resistor on the back of the gauge can. You may be able to fix it with washers under the resistor retaining nuts.

You can test the sender without getting under the truck. You'll need a good ground and a multimeter.
COMPLETELY fill the tank.
Pull the fuel gauge out of the panel... Remove the dash bezel, gauge lens, gauge bezel, and fuel gauge.
Set the your meter to 20vdc.
Turn the Ignition ON.
Plug the black wire into the ground clip of the empty fuel gauge pod. GND is the top or bottom of the triangle.
Find the 12v feed clip by touching each of the remaining clips. One will make the meter read battery voltage. The remaining clip is your sender lead.
Turn the ignition OFF.
Set the meter to 1K ohms and make sure it goes to zero with the leads touching.
Plug the red lead into the fuel gauge sender clip.
The meter should read near @ 90 ohms... 85-95 is good enough.

If it doesn't read properly you have a damaged sender ground, damaged sender wire, bad tank switch (73-80 (81?? 82??), or bad sender.