I'm currently having real issues with my Tivo, every recently recorded program is jerky during playback. I tried deleting some programs in case it was down to the drive being quite full but this hasn't cured it and I've tried switching off and rebooting, all to no avail. Watching TV is fine, although we have had some issues with flashing lights and loss of service occasionally.
I'm not convinced that it's a signal issue as watching live has no issues it's only the playback of recent recordings. I sort of came to the same conclusion as you in that I think the hard disk is on its way out. Annoying since there are quite a few films I've still to watch but it seems like a phone call will be in order.