I am in the military overseas trying to help my parents out with their Tivo. I saw a post very similar to the issue they are having but it wasn't answered.
The issues is in the last month, the Tivo seems to lock up and eventually reboot when they view or record HD channels. Non-HD channels have no problems at all. By "lock up" I mean the picture sometimes freezes, the remote becomes non-responsive, and sometimes they just have a black screen. When the remote does respond, sometimes the menus take an extremely long time to populate, or to transition from one to another.
When recording, the Tivo records the HD show in a series of "partials" with significant gaps between them. Sometimes it reboots as they watch - again while either they are watching an HD channel or Tivo is recording one while we watch something else.
The internal hard drive has been upgraded to 1TB but is not making any noises (screeching or popping).
Time Warner Cable was in the house a couple of weeks ago and changed out the CableCard so that isn't the issue.
The problems I can see that it might be are:
Bad hard drive - although it seems strange that SD channels will record and play just fine
Signal Strength - although the TivoHD was working fine for 4 years until recently and all on the same cable feed
Any suggestions would be appreciated.