As with any pixellation fault.... first reboot your box.
If that fails to resolve, then you have a fault somewhere, and that's causing poor TV signal. There's 20 Sky Sports channels, if you can be specific about which ones (including SD/HD?) then we may be able to help narrow things down a bit. It probably won't change the end result...
Call VM on 0800 5610061 to check if there's any reported issues in your area that could cause this. If not, then you are advised to call Virgin on 150 (or 0345 4541111 from any other phone) to report your fault. It will likely need an engineer visit (Covid issues not withstanding) or a replacement box
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