I'm going to agree with David C Klasse, my 95 C280 is rock solid at all speeds. I've got it up to 125mph. The only thing extra on my car are 17" AMGs with performance tires.
I would check control arm bushings. However, a 98 with 42k miles sounds unlikely. Just double-check all of the suspension parts. Has you car always behaved like that or is it the first time you got it up to those speeds.
I'm real curious why a lot of you are having problems. Mercedes is known for its rock solid stability no matter what the speed.