It sounds like a worn steering box symptom. This is common in hi mileage MBs.
I have had to replace two in my W123's, one was really loose when I bought the car in 1990 so I went to a wrecking yard that happened to keep records of the mileage shown on specific items (from the car odometer before they scrapped the car) Luckily they had a low mileage unit, it fixed the problem cheap.
The next time I needed a steering box for my '85 300TD I couldn't find any units that felt tight. Worse yet, none of the wrecking yards showed mileage figures so I decided to send my box for rebuild. I have been very happy with it. You might try Meridian Auto Parts near San Diego tel 888-275-2525
http://www.meridianautoparts.com