My standard answer to pixellation type posts is as applicable to this query as anytime, with a few specific comments. Your TV's inbuilt Freeview tuner isn't using VM's cable network, so your observations here only prove that your TV isn't causing these issues.
Pixellated pictures on live TV are often the sign of a signal fault. If you've rebooted your box once, then there's likely a fault somewhere, so this will likely require VM intervention. You're likely to need either an engineer visit or a replacement box, depending on what VM's remote diagnostics turn up.
Live TV doesn't use the internet, so your broadband speed is irrelevant for pixellation issues. You've mentioned having recently joined VM - was this a manned install, or a QuickStart self-install? If it was a manned install, I'd have thought signal levels should have been checked. If you were self-install, then this potentially hasn't been checked.
Depending on the answer to this question, the next course of action can be advised.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks