I appreciate that this thread appears to concern only automatics, but could it in fact be nothing more than a slightly dirty Idle Air Valve, showing a tendency to stick slightly open when shutting down to Idle?
(I presume that an automatic works in the same way regarding the engine - ie does it have an IAV?)
I periodically have exactly the same fault with my manual gearbox version, and on every occasion it responds to a carefully cleaning of the shutter mechanism in the IAV using a small brush and carb cleaner.
It has been doing it a bit with the hot weather. Yesterday I decided the time had come for another clean, but then it cleared itself.
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