eratic downshift when at cruise speeds

the engine was just rebuilt and when you are cruising it seems like the tranny downshifts without exceessive pedal travel.not reaaly trying to make it downshift to pass...otherwise it shifts normallyor maybe takes a little longer to shift on accelleration when accellerating from a stop and if you stomp on it it works ok. is this possibly a linkages problem or a modulator ? i thought maybe linkage because of the engine being removed for rebuilds
other than the erattic downshifting it seems to be ok on accelleration and decelleration....its an 86 buick century.