With age and use, sometimes the shuttle valves might get varnished or gummed up and if they don't move as they should (manual selection plus fluid pressure against spring pressure), the shifts don't happen as programmed. In the old days, the best cleaner was Berryman's B-12 in the ATF (as stated on the back of the can) and in more modern times there are now other additives to clean things up (GM has a great one and Chrysler probably has similar) and also swell the seals slightly.
I remember that the Berryman's would turn the fluid black in about 30 minutes of use, prior to a full fluid/filter change operation. Even on transmissions where the fluid was not all that dark already.
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