I have a 2007 ML500 with I believe the same transmission behavior. I have taken it in several times for the transmission but there was little they could do to resolve this. I talked to the service technician on one occasion and he said that the hunting for the gear is normal in this transmission, mostly at the 1 to 2 and 2 to 1 gear shift. They updated my software once and I had the transmission service done at 41K miles with mostly same behavior.
I test drove a new 2010 GMC truck with the new 6-speed transmission, lo and behold, I was getting the same hunting for the right gear behavior. I did not buy the thing and decided I would rather drive my 98 GMC with the 4-speed auto.
In my opinion - The new transmissions are putting "more" gears in order to be more efficient but the engineering is not there yet to make them "smooth" operating.
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