The dreaded line just showed up on my hd channels after the latest system 'upgrade'. After a bit a research online I figured it was an issue with the old style box (i've got a newer model in another room with no problems at all). Two boxes later, I think I have solved it.
First, I had to figure out how to get into the mysterious and elusive "stb" menu. Easy enough if you know how to find it. Just hit the 'menu' button while the box is off.
Once the 'stb' menu appears, goto the 'additional hdmi settings' line, and change the color space to rgb. Turn the box back on and presto.
That's it. It literally takes less than a minute to fix. Now why was that answer so hard to find?? Hopefully this post will save someone the time/trips to comcast/frustration that I've experienced.