97 Galant auto trans problem

1997 Galant ES auto 94kmiles 2.4L. When I drive the car in the morning, the transmission shifts fine. After 20-30 mins of driving the shift becomes hard and revs up. It happens during the day if I drive the car within 2-3 hours of last driving. It specially get worse if I put it in park and then drive (engine running). However, if I do not drive it for 8-10 hours (or the next morning), it shifts fine. My question is, if it is a sensor problem (getting hot) or fluid problem (over or under- when I checked the fluid level last time it was a little over) or rubber seal problem (not sealing properly) or anything else ? During my last trans fluid change, there was no metal pieces, so I assume gears are ok. Thanks for any sugestion.