I've got a similar issue with my '87 wagon. Haven't been chasing it too hard because I've been busy with the non-Mercedes parts of my life!
I've got a steering wheel vibration which comes & goes. Mostly it's not there, but every once in a while it appears. Highway speeds only - greater than 60MPH. Unlike everybody else, I already have had a GSP9700 balance done on this car. It passed with flying colors. The car definately was smoother following the balance - but the intermittent vibration remains.
Personally, my bet is on lower control arm bushings.