This sounds similiar to what mine was doing. I accidentally overfilled the transmission with fluid which resulted in a lot of the fluid settling to the bottom of the torque converted after shutdown. This caused the converter to "engage" as soon as I started the car and it would jump forward slightly then settle down.

After changing the filter and gasket and putting in the correct amount of fluid it works like a charm (and shifts alot smoother).

Hope this helps,
Tracy