I've read about older boxes doing this ... usually a hardware problem (tuner/chips going bad).
http://www.tivocommunity.com/tivo-vb/showthread.php?t=371500
http://www.tivocommunity.com/tivo-vb/showthread.php?p=5717738#post5717738
http://www.tivocommunity.com/tivo-vb/showthread.php?t=371500
http://www.tivocommunity.com/tivo-vb/showthread.php?p=5717738#post5717738