I think I finally figured out why some of my channels occasionally go blank. It occurs on nights when there is an Emergency Alert System test. Doh! Is this a known problem that I've been slow to pick up on? Since I am usually never up that late I do not even know when the tests occur. I have seen recent posts indicating that the Tivo will now go back to recording the channel that it was on before the test, but the channel that the other tuner happens to be tuned to appears to go blank.