Are you using HDMI or RGB? If your using HDMI try RGB cables there has been many bugs with HDMI and some TV sets.This will determine if it's something wrong with the cables or the actual signal.

Try different cables first, that's the easiest way to trouble shoot. Try another HDMI, try RBG, and see if you get the same answer. If you do then call At&T and see what can be done.