2000 Lumina - AutoTransmission hard shifts after warm up.

I bought a 2000 Chevy Lumina a couple months ago and it has been great. I took it on it's first long trip the other day of about an hour drive. When I got off the freeway it started shifting hard on me. speeding up you can really feel it shifting hard especially if accerating quickly. It also is noticable when slowing down.

Since I have returned from my trip it has not had this issue so I went on a long drive and about a half hour into the drive it started shifting hard again.

anyone have any clues to what may cause this? I'd hate to have to get a new transmission after just buying this car.

-=Mark=-