I have a problem rewinding live broadcasts

I have had TiVo on Comcast for a few years, and I had no trouble with this. About a month and a half ago, I got a new TV and a new connection. I don;t know if this is a coincidence, but my TiVo started malfunctioning. If I want to rewind a live broadcast, it will only go back 10 minutes. Once I rebooted, and it seemed to go back to normal, but soon it was back to the 10 minute limit. I had Comcast change the box, and I have the same problem. It also rarely records suggestions, and, if it does, it deletes them almost immediately.

Can anyone figure out anything about this?

 

No, this started when the new box had nothing recorded on it.

 

I just experimented and found out I only have the problem on HD channels, and the new TV I got a month and a half ago was my first flat screen HD TV. This must have something to do with the problem.