We have virgin V+ HD box connected by HDMI to a Samsung LCD TV. Every once in a while we get random pixellated lines on the TV and the sound is affected at the same time. We have fitted a new HDMI lead, tried a different TV and checked the signal lead to the V+ box is tight both ends.
This has started since before Christmas and is getting progressively worse over time.
Could be a signal level issue that would need adjusting by an engineer but as you've got an old V+ box I'd be more inclined to suggest the tuners or the HDD are failing. As these boxes are no longer supplied, the replacement would be a TiVo or V6 with the latter only supplied if you also have broadband with Virgin.
You can either ring up 150 / 0345 454 1111 and select "if there's a fault with your TV service" to get them to run the remote diagnostics or you can wait for the forum team to do this but they will take around a week's time.