I had this problem with my sons 91 300sl. We dropped the pan and found an element type filter that was very dirty. We replaced it and the trans came back to life but was slow to shift. I attributed this to worn or thinner clutch packs. We sold the car soon after and the guy that bought it said he found the problem. It was something simple but I can't remember. I think he found the solution on one of the Benz forums.