Assuming you've rebooted your box once as a precaution, and all your connections are finger-tight, your pixellation & picture breakup will almost certainly be caused by a signal issue which Virgin will need to rectify. That could be a faulty box, it could be a cabling issue - but either way, reporting it to VM is the way to go.
Are there any specific channels (HD or SD?) that are affected, in the meantime?
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