strange shifting

I have a 94 mustang GT conv. with an automatic. When I take off normally from a stop at about 20 MPH it feels like the trans shifts from 1st to third, hard enough to make the engine bog down. But if I take off fast it feels like theres a shift kit in it. Then when slowing back down to a stop it doesnt downshift until almost at a dead stop and nearly stalls the engine. is it possible my torque converter is locking up to early and not releasing when i stop. or is there an electronic problem. Thanks in advance for any help!!