I'm so sorry to hear that you've been experiencing some issues with your TiVo® recordings, I totally appreciate how inconvenient this must be for you as I had a similar issue last year.
I've done some testing on your TiVo® and can see some issues with the hard drive as Parrotperson suggested. I'd like to arrange for a technician to call out and investigate further for you, so I'll send you a private message in order to get the ball rolling with this.
Please look to the top right of this page where you'll see a purple envelope icon, click on this and you'll see my message.
Many thanks and apologies once again.
P.S. It's very likely that the engineer will swap the TiVo® for a different one, whilst we are unable to save any of the recordings, there are things like series links, wish lists etc that we are able to back up. Why not check out http://virg.co/boxswap to learn how to back up some of the settings you already have.