But, what channel were you on? I've seen, several times, people end up on SD channels instead of the HD channel. Since cable boxes carry both SD and HD channels, you have to be sure you are on the HD channel to get the best video quality.
Another thing to try is a proper HD source, like a Roku or similar device. Perhaps a PC or Laptop hooked up to the TV and playing some HD video off of YouTube or from Netflix. This way you aren't just trying one thing, and complaining about one thing, then finding out that one thing is the problem. Not the TV. I expect if you hook up a laptop directly or a Roku (or similar) that you may see a marked improvement compared to the cable box, and that it may be something about what is going on with the cable box itself.