I've had similar problems on more than one (older) car and I think it's caused by the wiper not riding over the lower end of the wire wind, which allows deposits to build up over the years. I expect it's more likely to happen if you are in the habit of filling the tank when it gets a little below half as I am.

I tried an experiment on a 1000 mile trip with the 2.6 where I just filled the tank to about half and then let it run to near empty, so the wiper spent plenty to time passing over the lower portion of the wire wind. Had to stop for fuel more often than normal, but as the trip progressed the fuel gage stopped dithering and has not misbehaved since that trip last Thanksgiving.

Duke