My 95 E 420 with 46,000 miles is not downshifting from 4th to 3rd when slowing to go around a corner, or for traffic. After slowing down, to 15-20 mph and then accelerating normally the trans downshifts harshly with a clunk. If I downshift manually, it drops into third smoothly and I can accelerate normally, and then manually shift up to 4th. When stopping completely the trans downshifts through all gears normally and starts out in 2nd. TYIA for any ideas.