Check your fluid level. Is your D light flashing? Is you cel on?How many miles on the car? Your last fluid change?

Do some research on stall testing trans.,shift solenoids,clutch pressure control solenoids and psi switches here and the web.

This will give you a good Idea whats wrong and how a/t work.

It could be a clutch pack going bad, shift solenoid or clutch pressure control solenoid going bad. The stall test will tell you more.