Listen to psfred. Never let anyone machine flush any transmission. I have a good friend who is an independent transmission man and says that his business increased a lot when the "Flushing machines" came into town.
What happens is during the flush operation, fluid is pushed back through the filter and dislodges any trash in it. This trash then gets in the valve body and the trans is history. Best advice is have the trans serviced correctly (drain converter, pan, and change the filter). If the fluid is dark, then change it (and filter) again in about 500 miles.

Keep it clean and it will stay happy for a long time.

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84 500 SEL (307,xxx miles)