Try a search on this topic, there's TONS of info. Very common MB problem...many cars seem to develop this problem. I even contacted VDO (the makers of the gauges) about it and they denied there is a fault on any of their gauges.
There's been two main suggestions -
1) Fuel gauge sender unit - as mentioned above.
2) Gauge itself faulty - bad solder connection, etc.
You can try removing and cleaning the sending unit (search for instructions on how to do so, there's plenty), this sometimes fixes it. You could purchase a new unit which might do it, however it could also be the gauge.
FYI...all of the Mercedes I've owned have had this problem, from my 1971 250 sedan to my current 1989 300E. Not sure what I'd do without my trip meter...