I had a similar issue last week. Several channels were pixelating. System resetting, powering off/on of TV box didn't help. Called support and they remotely did a system reset/refresh which didn't resolve the issue. Had me change my HDMI cable which didn't help. Suggested to exchange my TV box which was a plan, but as a last resort the agent 'boosted my signal' and the pixelation went away and has been gone since. Call them and see if they can 'boost your signal'!
Those were the terms the Comcast agent used, hence the single quotes. Whatever was done, immediately resolved the issue for me.
Andrew is correct. There is no such thing as remote signal adjustment yet. It takes boots on the ground at the lines.