Use component cables instead?
I suppose it could be from a variety of things. I've noticed distinct differences in picture on some tv/811 setups depending on which resolution you use. I think DVI on the 811 is 1080i only (I may be mistaken here) so you might want mess with your tv and 811 settings and see if anything changes.
If not, the 811 might be bad. Dish might ship you a replacement.