Had a similar problem with my '88 190E 2.6 last year. It developed a vibration that I eventually traced to a tire out of round situation, but in the process of doing some testing the car went into a violent front end shimmy, the likes of which I had never experienced on ANY car. I knew the steering damper was seeping, so I pulled it off, and it offered virtually no resistance.
I purchased and installed a new damper and a check of the 205/55ZR-15 Dunlop D40M2s on a Hunter 9700 showed excess radial force variation on three with the fourth marginal, so I got an adjustment toward a new set of Sport 8000s in the same size being as how the D40M2s were out of production. The forcing function of the tire radial force variation combined with the worn steering damper was enough to set off a runaway front end shimmy. The stiff sidewall 55 series tires relative to the soft sidewall OEM 65s could also contribute to such a problem, especially with a deteriorated damper.
Mercedes have a lot of front caster (about 10.5 degrees on my 190), and apparently a steering damper, which doesn't last forever, is absolutely necessary to damp out runaway oscillations. As another example, sixties vintage Corvettes have a steering damper, except those models with mechanical lifter Special High Performance or Fuel Injection engines. The longer deep section of the oil pan on these engines precluded installation of the damper, so GM just left it off, and it doesn't cause any adverse affects in the steering system. However those cars only run about 2 degrees caster.
I think the damper cost about forty to fifty bucks and it's easy to change - just two bolts, but you have to remove the belly pan first.