Check the FILTER!!!
I have a '84 500SEL with Full hydropneumatic suspension. When the car suddenly started acting up, I came to this forum and got nowhere. No help whatsoever!! I finally solved the problem myself by discovering that the filter in the suspension system was CLOGGED. A lousy $2 filter was all my problem. Although your car only has this suspension in the rear, it must have a filter. Find it and replace it. Wouldn't hurt to flush the hydraulic oil(special oil available from the dealer- I bought it in CA for $6 a bottle from parts house). Then, see if you still have problems. Hard ride is the gas cells themselves, no big deal. Shocks usually don't go bad, mine are 18 years old and 300,000 miles-still perfect! Good luck, let me know how you make out. If you can't find the location of the filter or filters, (one may be a tiny screen type, which looks like the old filters in a carb) I'm not sure, I'll look in the factory manuals, I have a full set.