Why does my car studder?

Its a 96 olds cutl supreme 3.1L, when going up hills staying at a constant speed, usually over 50, the car jerks, and the rpms jump back and forth from 2000 to 2500 quickly like a bucking bronco. Does anyone know what this could be, i have taken it to several mechanics who all gave me different stories.none helped and i recently replaced the , but it still does it.