Sounds like there is a fluid pump pressure problem or a bad solenoid.
Have the trouble codes pulled to see if the problem is electrical. Any Auto Zone or Advance Auto will do this for free.
If nothing comes up you are probably looking at a rebuild. If you do so, go to a Transmission shop and not a standard repair facility as they generally send out transmissions to tranny shops because there are so many special tools and it also takes a good technician with a lot of experience with automatic transmissions to work on them. They are tough to work on. Don't take it to an AAMCO however. They are overpriced by about $500 on average and are such a mass production place I don't know if you can get the quality you can by taking the time to find a good shop.
The search engine on this site will help you find a reputable transmisson shop is near you.

Answered by DrCranknWrench (expert)
3,292 answers provided