About 4 times in the last 30 or so days since I bought it, my Tivo HD has lost the TV signal. I can still change channels and whatnot, but I am presented with a grey screen during normal TV watching. This is on analogue TV, no cable cards installed. It has recorded for the full length of program and I go to watch and am presented with the blue TiVo screen with a playback bar at the bottom with no green in it. Only a reboot seems to fix this. Is this a common problem?