The gas guage is likely the fuel sender unit located in the fuel tank. It seems to be a relatively common issue for MB's (both my 86 190e and my 94 e320 have this intermitten problem) You can order one from the fast lane for about $75 and replace it yourself. On the 190 it was located just behind the trunk carpet/plastic panels on the fuel tank. I doubt the C-class is much different.