I'd recommend you do a posting on your car probs...you'll get a bunch of quality advice I assure you, and you need it becasue NO MBZ should be unstable at a mere 100MPH!
Without knowing the mileage of your car, regardless I'd suggest that you start with a fresh 4-wheel alignment and then re-balance your tires (and personally I'd watch them do it and assure no rims are bent). Neither hurts, and you can establish a baseline as to where you stand.
If after the alignment/balance you still sense instability, bring it to either MBZ or a reputable independent and have them diagnose your problem...beyond what I've suggested, you can nickle-and-dime yourself to death buying the wrong parts; better off to know exactly what needs to be done.
Also, if your car has a steering damper I'd replace it...it's essentially a shock absorber for your steering system, and is a common wear item.
BTW, multiply miles by 1.609 to get KPH :-)
Best regards, Michael