I bought a low mileage 1990 300SEL this spring that was in beautiful condition, but had no service records. I had the car for less then 2000 miles, and was in the process of catching up on some regular PM work when the transmission suddenly refused to up-shift. First I felt sick, then I decided to do a transmission fluid and filter change, as that was on my list anyway. Well, I changed the fluid, which was at the correct fill level and kind of dark, and the filter. It corrected my problem. Apparently the filter was clogged enough to restrict fluid pressure causing the no up-shift. As I said, the fluid was at the correct level, and I’ve seen worse as far as the color goes, but that filter was in bad shape. Maybe you’ll get as lucky as I was.
Good luck