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There are a number of things it could be. With 70k on the clock, you're likely to need a filter/fluid change - if the fluid level is low (no way to easily check), this could cause shifting problems, esp. at high rpm. When you look under the car, is the gasket line of the tranny pan leaking? If so, you're will be down on fluid... how much is anyone's guess.
It's not likely to be the selector switch - how does manual mode work?
It could be the hydraulic control unit, but this goes beyond DIY for most people.
I would change the fluid/filter and see how that affects it's operation.
Beyond that, get it to a dealer for a proper diagnosis since there are so many possibilities for what you're experiencing.
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