2000 Oldsmobile Alero Question: Tranny shifts out and into gear while maintaning speed.What may cuase this?

My tranny shifts fine in all gears and when thorttle down it goes fine but when i am at a cruze speed the tranny slide in and out of gear but if i put gas to it she will go into gear and go. doesnt jerk into or out of gear the only way i know is by the rpm. If i go a distance it will stop it for awhile then drag car down and then smell tranny fluid burnen. I have changed speed sensor, and nuetral safty switch sensor.