pixalation

I have been getting pixelation on almost all channels specially on national channels of 2, 6 and 8.   Checked my connections and they are secure. Not sure what has caused it and the automated check on my network box from comcast shows no issues. I am ready to replace the box as a last option but I read that some folks have done this and had no effect.  Xfinity???? anything you can do to help resolve this I pay over $300 a month for all your services and really prefer to see the Soccer games without interruptions.