Pixellating TV pictures, assuming you've rebooted your box once, simply means there's a fault somewhere that's causing poor TV signal. If there's any specific channels you can identify as being worst affected this can sometimes help, but it won't alter the end result...
You are recommended to call Virgin and report your fault. 150 from a VM landline, or 0345 4541111 otherwise, and follow the options for "I have a fault with my TV service". You're likely to need either an engineer visit or replacement box - Virgin can determine which.
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