Then change the plug connectors under each coil...common misfire fault.

..I would also make sure the correct plugs are in there while you are at it.
104's don't like many plugs b/c of the Waste Spark Ignition design, but we have found that they DO like F8DC4/Bosch.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU21T0H5B5L&year=1997&make=MB&model=C-280-001&category=F&part=Spark+Plug