[BeZerK2112]

I had the exact same problem! The problem was that the previous owner never changed the tranny fluid.

Check your fluid to see if you see any particles floating around in it. The way the shop explined it to me was there was a rubber piece that needed to be lubricated that, with the old fluid low, deteriated. As the car heated up it expanded and made tranny work properly. I ended up replacing the tranny myself for 500 bucks. Perchased it and installed it. Worked great.
Good luck! If you have any questions just ask.