said by JPL:
Yep - good advice. Had the same issue with 2 HD channels a while back. They were the only 2 to pixellate, but because they were only breaking up on one TV, I knew it couldn't be the ONT. I checked the connections behind the TV - cable to splitter, and from the splitter to my DVR and my router. Sure enough, the coax connection going into the splitter was barely loose. Hand tightened it and the problem went away.
Also discovered pixellation on VOD on my main TV this morning (the only thing more annoying that Yo Gabba Gabba, is watching it while the picture keeps breaking up). It was fine last night, but not this morning. Reset the router, and the problem disappeared.
I've mentioned this before, but the reason only a few certain channels can be impacted by loose or corroded connections (or bad splitter for that matter) is because of the frequencies of the QAM channels on which they reside. It's a physics thing.