I have the same problem for weeks now with HD BBC2 and Channel 5. I called Virgin and was told it '...might be because of the hot weather' and they were 'only aware of a problem with Channel 5'. Well, the hot weather continues as does the (really annoying) picture break up so it will be interesting to see what happens when it cools down again
I have a TIVO box downstairs and a V6 box upstairs. The HD channels on the V6 box are absolutely fine but a number of HD channels on the TIVO box have picture break up. Those channels include most of the Sky Movies HD channels as well Sky Sports football HD and Sky Sports cricket HD. There are also other HD channels including 4HD that also suffer picture break up.
Update: My thanks to Louise for organising an engineer visit. He came yesterday and changed the connectors, but mostly the signal strength was fine and he couldn't find any 'faults' per se. He also said it could actually be the very hot weather causing the picture break up. To be fair, this now seems to have resolved itself on BBC2 and Channel 5 but is now affecting ITV4HD and BBC4HD. I'm not wishing summer away of course, but it seems things may get back to 'normal' once the weather does!