If it isn't overload, then it must be trees. I don't know your exact address, but I see a lot of trees in your area. Trees block TV signals.
http://www.hdtvprimer.com/antennas/siting.html
scroll down to Trees and UHF
The only other thing I can think of is interference to TV reception from your strong local FM signals.
http://www.fmfool.com/modeling/tmp/6...e/Radar-FM.png
Please let us know the results of your tests on this same thread.