I have replaced the fuel sending unit for just this sort of a problem. In my case, the gauge seemed to get jumpy as fuel level got low. Since the sending unit is a float-mounted sensor that rides up and down along a wire and changes resistance with its position, there's some sort of electrical anomaly that begins to happen as the sending unit ages and wears. Someone else may know if there's some other easy fix, but I simply replaced the unit to correct the problem. Someone else mentioned a cluster potentiometer. The only pot I know of in the cluster has nothing to do with the gauges, only controls cluster lighting.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't