Speaking from experience, before I would do any serious transmission work, I would buy a bottle of Trans-X. Many mechanics swear by it. You probably have a stuck valve in the valve body. This happened on my 420SEL. I put a bottle in the tranny, not long after that, it turned back to normal, and to this day have put 60,000 miles on it, with no problem. Give it a shot before you dump a ton of money into it. And I would have a mercedes specialist tear apart your tranny instead of a generic shop.
93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K