the tbird i just bought does the same thing . i got a cuz who worked for a tranny shop forever before leaving to work in the shop at coca cola , and he told me { mine is like yours , don't look like a thing has been touched since day one !} that drain the tranny and convertyer , change the filter , as you said you've done elsewhere , and hopefully it'll come out of it . seems according to him that the fluids were let go so long and got so contaminated that it messes up a sensor or something in the converter that controls the locking and unlocking feature of it . if not he says it's time for a converter at least .