I'm not too familiar with automatic Nissan transmissions but I am around Honda autos all day where I work (Honda). There is no real fix for slipping auto trannys and the solution is to just install a new/used trans. You can try a flush and that might help a bit, but like stated before autos were never designed to hold up to the wear and tear of a 5 speed and rebuilding is usually not a favorable experience.
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