Mine is doing the same - also a Samsung box. Initially the picture freezes, and the sound continues for a few seconds before that goes too. Changing channel and then back again clears it, but only for a few minutes before it freezes up again.
Have rebooted the box but that hasn't fixed it. Have also tried to run a diagnostics check via the website, which gets about three quarters of the way through but then fails...
Assume this is a service problem, that Virgin haven't yet spotted.
I'm really sorry to learn about the problem with your TV service lately, I've not been able to accurately locate your account to take a closer look at this for you unfortunately.
So I can run some tests on the line for you, could you respond to the private message I've sent with the requested information? You can view the message by clicking the red envelope on the top left of this page.