Over the past year I've had it where out of no where there is a black screen with just sound after having a perfect picture with issue. This has happened about 5 times in the past year.
After a fair few restarts of the Virgin v6 box and a play with the hdmi cable it comes back.
First time it happened I swapped the hdmi cable. But now its happened enough that I know a few restarts solves it, and some times a play with the cable. It's possible it's the hdmi port on the box, and on top of that something with the box itself.
It's extremely annoying when it happens. I would have an engineer out but I can garentee when it happens. But I know its only a matter of time before it goes completely.
I have seen on lots of the Virgin forums that other people have had this and had the box replaced.