Rough transmission on Mercury Mystique

My wifes 98 mystique has a slight jerk when the transmission changes gears. Sometimes it's very jerky and sometimes it's smooth. The car has 79000 miles on it. Do we just need a transmission flush or could it be the start
of someting more expensive? Also, what kind of ballpark figure are we talking about to get this done?
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I believe Ford recommends a fluid change every 30000 miles or so. If it's never been done, do it NOW.
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Andrew Rossmann wrote:

Good idea (considering the mileage) to flush also. Been there.
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