chrysler grand voyager 3.3 le auto - grand voyager gearbox - fasthands1969

hi have a 1998 grand voyager 3.3 auto that keeps coming out of gear when i stop then when you try to drive off it just revs then jumps into gear but jumps out again when i stop again i have changed the gearbox fluid with atf4 and new filters as chrysler says to do but no luck there any ideas thanks