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When I first put up the antenna WGRZ signal was stable almost every day. (That was in cooler weather, back in April)
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That makes perfect sense. In warm weather, a temperature inversion is formed in the signal path. This causes the signal to be trapped in a high altitude duct, unable to bend down to your antenna.
http://www.mike-willis.com/Tutorial/PF6.htm
scroll half way down to
Example of a path profile diagram that shows how refraction bends the signal to your antenna
http://www.dxfm.com/content/propagation.htm
scroll down to
An illustration of a high-altitude tropospheric duct.
It is most likely to happen when the signal starts at a high elevation and there is a bowl between the two antennas where the inversion forms, just like what you have.