Sounds like the nicam bug that early ones had,now the questions =
1/ have you had the Tivo long?? - if not the software might not be updated to 2.5 or whatever the latest version is,this update fixes the bug in most cases.
2/ if not,a call to Tivo,who will arrange to have the unit`s nicam fault fixed so it`s bug free,there is no charge for this.
3/ hop over to the tivo forums,there is a wealth of information and friendly helpfull people just waiting
4/hope thats of help to you MrBlue