I have an '83 300D. The transmission normally shifted into 3rd gear about 40 mph. It has started shifting between 57 and 65. After it has gone into high gear, you can slow to 10 - 15 and it will downshift and shift back up at normal speeds but if you come to a complete stop it won't shift till you get up in the 57-65 range. Is this a vacuum problem, a modulator valve problem, or something worse?