1999 Toyota Camry Transmission?

Our speedometer will jump up to 50 mph at times. Most of the time, it doesn't affect the engine at all (no revving, etc) but every once and a while it kind of feels like it's stalling or slipping. We added more transmission fluid, since we checked and it was low. Also, the problem usually happens within the first 15 minutes of driving it in the morning. If we've been driving a while, it's fine. We're just trying to figure out if it's the sensor or worse and how crazy expensive this is going to be. :(