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