2001 Other Saturn Models Transmission slips when engine is h

I recently bought a Saturn SL2 as is and am running into a problem that I didn't have any idea was there. Whenever the engine is hot, the transmission shifts hard and sometimes even doesn't want to shift into overdrive. It mostly seems to happen after the car has been running for a little bit (like 30 minutes). I have already changed the transmission fluid and filter and that seemed to help a little but but there is still a problem. One of my friends thought it might be a speed sensor and that somehow this tells the transmission when to shift. Any suggestions?