After 2 years of working perfectly, my Tivo box is glitching on BBC1, BBC2 and ITV, plus E4 refuses to display. The HD versions of these channels are fine.
I have already disconnected and reconnected all of the cables and unplugged/replugged in the Tivo box all with no improvement.
One piece of information which may be of use is that when I take a look at the diagnostics page, the signal strength is shown as 66%. I don't know if this is an acceptable vlaue or might be contributing to the problems?
I would say 70%+ is what you should be expecting from a closed system. Try some other channels. If other channels are giving you 80-90% then something in the line is causing resistance to increase & needs looking at.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.