If the problem your describing looks like lots of little square boxes appearing on screen about 1/2 inch wide then the problem is probably a low signal to noise ratio, (ingress or interference can also cause this but it's not as likely).
If you have a satellite dish feeding your TV try repointing the dish to get the best signal possible.
If you have cable then you may require one of two things. Either install a small distribution amp where your cable comes into the house but this can be tricky to set up unless you have access to a signal level meter and some attenuators otherwise you could end up overdriving the TV's input. If you do have access to all of this then set up your system for between 0-5dB input at your TV accross all channels.
The second option, and far more simple, is to call the cable company and explain the problem!! They'll send out a tech to verify and adjust levels in your home or outside in the cable plant.
These are merely the simplest and most likely causes, if the problem persists, post again!
Hope this helps,
Terry
CEO/Owner
Beyond Home Theater
www.beyondhometheater.com
Two of the 14 HD channels we get have slight interference. Maybe twice during an hour show there will be some breakup in a small area of the screen. We have never seen this on any of the other channels. The set is an LG 50PX5D.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic