im no profesional nor a mechanic  years ago i had similar problem with a vauxhall carlton  i had the gearbox oil changed with new filter that seemed to work if i remember right but from memory there was something later wrong with the torque converter  thing with auto gearboxes is no one wants to touch them that ive found  1 reason i was given was they are built by magicians and worked by witchcraft !!lol  i wish you luck hopefully someone else will have a better explanation and can help you better