Ok, a rough idle could be caused by many things... lets go over the basics.
When last were the spark plugs and the plug wires changed?
Are you using quality gas, and 92? Try a bottle of TECHRON in the fuel system.
When last was the fuel filter changed? This caused a rough idle for me.
What about your air filter?
These are the common, more likely reasons for a rough idle.
But if you would tell your model car, year and mileage, it'll help us out further to assess what else could be your problem. GOod luck.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)