Hi Bruce, can you reproduce the problem by wriggling the wires from the sender to the gauge, check all along the whole length of the wire,and is there any fluctuations in the gauge reading while you are taxiing, I would be looking at the connections at the sender for a good tight clean contact, and if you can get the problem on the ground, earth the sender wire to see if the gauge moves, if it does not then I might suspect the gauge as being faulty, if it does move then I would suspect the sender unit
Brian