Since I have the German 'autobahn' real nearby, I do a lot of high speed rides with my C250D. I do a lot of 180-210km/h (112-130mph) long trip traveling and have to say that the car IS stable. My C class has driven 375.000km and I have recently renewed the complete front suspension.
I drive the car with 15" winter tires and 17" summer set-up without any wandering problems.
When there is a LOT of wind, however, the car becomes fairly nervous at high speeds, but I think that is pretty normal...I experienced the same nervous wandering at high speeds and hard wind with my fathers 1999 E220CDI and my 1990 300SL.
It is not normal for a C class to wander at "low" speeds!
As far as the possible solving of the problem goes, I have to agree with the facts stated by JS