Seeing pixalized frames when demo'ing flatpanels. What is it?

I'm trying to decide on 42" flatpanels. After viewing many sets on a HD source in Best Buy and Circuit City I have a question. Exactly what is it that causes the picture to look like square pixels for an instant and then gone? I noticed it on many sets during a fast-motion scene or a shift between scenes. The customer next to me didn't know what I was talking about when I pointed it out so maybe I'm just crazy. I have cable HD (and have had satellite HD) on my rear projection HD set so I'm quite familiar with big blocky pixels when the signal gets weak or goes out, this is not the same. I don't know exactly how to explain it, it's like the entire screen becomes highly "digitized" for a brief instant and then gone. What is this called and is it from the source or the TV? If it's from the TV, what attribute should I look for in a flatpanel that will minimalize this? Thanks.


Rob