1995 Ford Taurus Frequent shifting and rough idle

At about 35-40 miles per hour my car will shift frequently back and forth between gears. This especially happens when going at slight incline or slowly accelerating. Also the idle is rough, it feels (no tach) like it keeps dropping rpms on me, but it doesn't stall.
I recently replaced the engine computer because it was constantly reading rich on one O2 sensor and I also replaced that O2 sensor; it was probably driven for 3000 miles with this faulty code. The car has been driven long enough for the computer to relearn.
Another issue that may or may not be related is that the interior sometimes smells like exhaust and today the catalytic converter was giving off white smoke (it was a warm day and we had just driven up a steep hill). The check engine light does not come on.