how eratically are we talking? Series 1 or 2?

the series 2 are particularly bad for it, you tend to find going round corners one way gives you a higher reading, and then corners in the opposite direction show a lower reading.

If thats all its doing then its perfectly normal, in a straight line at constant speed it should give you a fairly steady reading, and is an accurate reading of how much fuel you do actually have in the tank