bonrich, it sounds like your system is the same as mine - Hunter Pro-C controller. The issue you are experiencing is caused by a second device which is behind the Pro-C controller.
The Pro-C is on a hinge and behind it you will see a Hunter SolarSync. This device senses atmospheric conditions and will increase or decrease the settings you have set in the Pro-C. You can find owner manuals at the Hunter website.
By the way, the only way I found to completely stop the increasing/decreasing is to disconnect the SolarSync.
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