First, that 1996 E300 is close to eight years old, and the mileage probably doesn't mean that much. The W210 cars have known steering and front end problems including front lower ball joints that are prone to wearing. Our 1998 E300 had them replaced long before 70K-miles.
Have you replaced the tires? Different tires have different driving characteristics.
Have you had a four-wheel alignment? Have it done at someone that knows these cars. Quickie chain shops have kids working under the rack, and that's the last thing you want.
On our car, we had the lowr ball joints replaced and a four wheel alignment done and it cured the problem along with tossing the junk MXV4 Michelins in favour of Bridgestone RE950's.
1998 C230 "Black Betty"