1988 Oldsmobile Ciera downshift problem

When driving at low speeds up to 40 trans downshifts fine. When driving on highway at 50 or above and then taking an off ramp and stopping trans does not downshift. Must do manually, otherwise car will "buck" and shut off. When restarting and placing in drive I must do this 3 or 4 times to be able to place in gear without stalling. Trans fluid and filter just changed.