...but any fast action on the screen turned my picture from a great looking glass into a sea of tiny pixel boxes, these pixel boxes more commonly called “micro blocking”
I'll steal this from Scott's article from MultiChannel.com. I'm not sure if macro- and mircroblocking are the same thing, and someone else will be able to provide a more technical explanation.
I seem to see this a lot on NBC's NFL coverage. Sometimes with CBS but not as bad.