I know this is an older topic, but just as a technical operation comment, you can have pixelation issues resulting from weak signal on specific channels that don't seem to affect other channels because the frequency of some TV channels are going to be different. Something that is affecting the low-end frequency spectrum might not be having any issues at all on the high end of the spectrum.
So for a real-world, hypothetical case example:
My point is that you absolute can have issues with pixilation with specific channels rather than all channels, and to find & fix this it will require a field tech to come out and investigate why (a troublecall, a repair service call that has to be scheduled). Sometimes it's a simply as a single bad fittings that just needs to be fixed, and then it's all good. Other times it's more complicated.