I don 't have the exact same problem but on both the hd-pro and 500 I have 1/16" diagonal lines from top right of the screen to the bottom left side of the screen. With or without hdmi setup.
occassionslly the picture clears up but then these lines return.
these lines appear to be analog a noise.
I took the sling hd pro close to 340 miles away with me on vacation. Had the same issue..
btw, I have Directv. This copyright protection thing apparently applies to component cables too.
It turns out that Comcast had degraded my signal from component. Hdmi was
fine when directly connected from my cable box. I have the m1 so component
only for me. The slingbox definitely enhanced the lines where they were
much more noticeable than when directly connected from the cable box. I
guess all I can do is complain to comcast.