Have you tried changing the fluid, making sure the level is correct?
I had the same problem before with one of my old cars. The problem was the tranny itself. had to get it rebuilt. cost about $1000. AAMCO. Pretty cool guys, they might be able to help. Good luck.
Question: Do you ever rev the car , then pop it into gear to peel out? If you do, I guarantee the ur tranny needs to be rebuilt. (I know from experience) Good luck.