Paul,
When I had this experience on my GTD, the first thing I did was check I the resistances at the senders (which checked out as ok). In MY case, it turned out the be some dodgy connections at the gauges themselves, along with one needle that was dragging just a bit on the gauge faces.
The advice posted here is good, but if your senders check out ok then clean all the contacts and connections before spending any money. It worked for me but as they say, your mileage may vary.
Mark