I have a Vizio 37 inch TV - connected to a Comcast HD cable box. Everything was fine unil this evening.

The cable TV picture now has a fiarily significant shift/flickr on the lower edge with a white line appearing on and off. The picture overall seems to shift. This is HD and non HD channels being played through HDMI. The picture is fine if I play the same cable input from the cable box through coaxial cable, and not the HDMI cable. I also tried different HDMI cables, and different HDMI inputs with the same issue. A DVD player connected via coaxial plays fine as well. I don't have another HD source to test.

It appears the issues are either with TV ability to play HD, or comcast HD signal.

Anyone have any ideas?

Did you try adjusting the aspect ratio? Usually this is done through a zoom button or format button. Something along those lines should exist on the TV remote, and possibly on the cable box remote. Usually you want the TV set to 16:9 and then adjust everything through the cable box.