I don't have an answer, but I have an 86 and a 91 190e and both of the fuels gauges bobble when they are sitting around the 1/2 tank marker and the car has just been turned on. They too settle down after some minutes.
A hunch is that it is the indicator in the tank. I have a vespa scooter that also does this when I first take it out of storage. After I shake the scooter side to side the gauge starts working. Not as easy on a benz.