2000 Breeze Shifting Problem

Anyone have any ideas what is causing my 2000 Breeze (85K miles) to continually shift between 4th and 5th at highway speeds? It seems that it
will settle into 5th when I get up to speed and then after a bit start downshifting, and quickly upshifting, for no apparent reason. Sometimes there is a noticable jerk and other times the shift is very smooth. At worst it will continue to do this at about 2 second intervals. Shifting into neutral and coasting seems to break the cycle for a little, but generally not long. Of course this is intermittent (10,000 miles or better since I first noticed it) and my mechanic has not been able to get a good read on it.
Thanks much.
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