Have you replaced your fuel filter? Mercedes recommends replacing the fuel filter and air filter at 60K miles. They also recommend replacing the spark plugs at 4 years or 100K miles (which ever comes first).
The fuel filter for this car is not cheap - it has a built in fuel regulator.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles