Another fault that shows up on ageing HD TiVos is memory. This caused by the memory chips on the board ageing and becoming heat sensitive in that they fail if cold. This shows up if the TiVo has been powered down for an hour or more. The test for this is to power up the TiVo for a while to warm up then power cycle it and see if it boots up. The fix is to replace the defective memory chips.
The best source for TiVo repairs is Darren King who can be contacted by message on here or at http://kingey1971.wix.com/tivorepairs.