My research has indicated that if gears 1-3 and reverse are working, the transmission is internally solid. There could be an exception, but it's unlikely. The o/d itself could be worn; this happens more often on manual trans cars when drivers fail to depress the clutch while disengaging the o/d and it wears prematurely. I would try a full flush, one in which the filter is actually removed from the transmission; "changing" the fluid only replaces about 1/3 of the fluid itself and leaves most of the dirt and sludge behind.