My 92 300d had the same problem. I consulted with the Mercedez Source website and he recommended trying Trans X ( additive), so I did. A couple months and a long trip later 1st gear came back. Oddly enough it was immediately after I disabled the EDS and changed the turbo waste gate actuator to a pressure operated one. It had to have been the Trans X loosening up a sticking valve in the valve body.