Are the devices connected to the tv via HDMI actually switched on when the problem happens? Badly screened HDMI cables can give out enough interference to knock out an aerial signal, especially if the HDMI and aerial cables run close to each other. Our service engineers at work are seeing this more and more often recently. Well-screened HDMI cables in most cases cures the problem. This is based on real experience of working for a Sony/Panasonic service centre by the way, not merely a theory. Might be worth a try?