Pixelization is almost always caused by a network speed issue. The Roku itself doesn't have a resolution below 720p, so it's not the display resolution causing it. Make certain you aren't having networking issues. Perhaps someone nearby has set up a new access point on the same channel you're using, and now you are getting interference.
Use a computer and go to speedtest.net and test your Internet speed. If it's under 10 Mbps, it's a possibility. If it's under 5, that most certainly would cause a problem. The question would be, what speed are you supposed to have and why is it not there?
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