Why don't you let a certified MB tech fix it? They won't guess at what the problem is (and let you worry about the bill ) they'll diagnose and fix it. Is your mechanic aware of all the service bulletins and transmission modifications that are available? Some of these trans mods are simple and cheap but you need to be able to do more than just bolt in a junkyard trans that may or may not work. Don't be penny wise and pound foolish. Let the real professionals do it right the first time and you can go back to enjoying your car.