1998 Ford Windstar Tranny?

My grandfather has A windstar and is very proud of it. It's been repaired countless times even a new engine was put it. Our family always tells him to buy a new van but he wont budge. Recently he backed out onto a steep hill and it was having some trouble later as we were driving the overdrive feature on his gear shifter kept blinking on and off and wold jerk as it turned on and off this has been going on for a few days and 3 different mechanics don't know why it is doing that does anyone on here think it is the tranny and if so should he rebuild, buy new tranny, or buy a new van

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