said by hofs1 :

You might have 480 overide turned on turn on everything then turn off dvr hit select then menu check 480 overide and turn it off make box output everything 720p and u should stop getting that problem. of course using component cables will eliminate problem but i cant see the difference in pq
i like 480 sett to output non hd channels in 480p then the tv fills the whole screen (stretched i know but i like it better than black bars)