When you say "Ch3 and Ch4" do you mean ITV-HD on 103, and Channel 4 HD on 104?
If you have pixellation on live TV this generally suggests signal issues and will often only affect the channels carried on the affected frequency.
If live TV is fine, try pausing it for a short time and watching it delayed - this uses the hard drive and is effectively a recording. Problems with recordings are hard-drive related.
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