fuel gauge not working?

Was a little perplexed when I last filled up my 2002 Jetta TDI. The gauge showed the tank was 1/4 full, yet the tank took 13.75 gallons of fuel (14.5 gal tank). At first we thought it was the pump at the station not working properly. Then I drove the car for over 150 miles and the needle on the gauge did not move. Then all of a sudden it dropped. It fluctuated like this for several days. I will fill it again tonight as I am at about 600 miles on this tank and it is showing the tank to be 1/4 full again. If I'm getting the same mileage as when I got the car a month and a half ago, the gauge seems to be a 1/4 of a tank off. Anyone else experience this? I'm annoyed because like I said, I've only had the car a month and a half AND if I hadn't decided to fill it at 1/4 full last time, I may have run out of fuel on the interstate.

The sender in the tank is probably going out. Most of these units have an arm with a float that floats on top of the fuel. If the arm for the float sticks or the resistor that reads its level is bad you'll get erratic readings like you describe.