I recommend locate the ANT751 up higher to clear the tops of the douglas firs or to a location that is not blocked by the douglas firs. If moving the antenna does not help then install a larger antenna , Winegard HD7698P antenna. If the plasma screen is still working Ok , Channel Master has a new tuner the CM7400.
http://www.winegarddirect.com ,
http://www.channelmasterstore.com