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This is a long shot, but consider this:
Get a can of carb cleaner and clean your maf and throttle bore (ideally take both out, and clean the throttle with a cleaner-soaked rag rather than spraying it directly). It might seem unrelated, but at 55k miles you're probably got some accumulated dust on the maf and oil soaked carbon in the throttle bore.
The transmission tries to adapt to the engine load, and the maf is usually the main signal for that, it doesn't have to be way off, but a dirty maf's error will be worse near idle and send a slightly wrong load signal to the transmission, which is where you're having your problem. Same with a dirty throttle, cars used to have idle control solenoids or stepper motors that would control an air bypass around the throttle to control idle speed, now that is done by the throttle plate closing further than it's neutral closed position into a finely machined section of the bore. Even if the engine computer can adapt to the carbon and correct it's idle speed to a certain point, it can cause the engine to behave strangely as the throttle plate hunts around the carbon "steps" to get the desired idle.
As I said, it's a long shot, but you're in it for the cost of a can of carb cleaner and 30 minutes.
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