I am glad to hear that cleared up your problem without having to go through the trouble and expense of replacing the fuel sending unit.
The cause for this must be either contaminated fuel, some additive that some brands put in their fuel or possible the way some nozzles deliver the fuel to the tank. I really am not sure if I understand it, but on two of my MB's changing fuel has cleared up the problem just as in your case.
For anyone else out their who has this common problem with Mercedes fuel sending units, I would first try changing your brand of fuel and running 2-3 complete tankfulls through to see if your fuel gauge clears itself up. Sure beats spending the time/money in removing the fuel sending unit and cleaning/replacing it.