1992 300SE, that would be the 5 speed 722.5 transmission. Always first check the trans. bowden cable, I believe that if you look below the front of the air filter housing you can see a plastic linkage piece that comes to a point, and it should be lined up directly pointing to a plastic 'nub'. It is possible that a very slight incline could be causing your downshift. Anything else and I would have to look it up at work. Does this 'new' car come with any used car warranty?