This just about the same problem I am having with my Slingbox AV with a cable DVR ... everything was working very well for a few weeks and then suddenly all I see is grey and white - no color. The video is in the upper half of the screen - and there are variable horizonal bars on the bottom half. Sometimes a warning comes on quite clearly - "Weak or no video signal detected."
I took apart most of the wires - and pushed and out - secured the S-composite video and other video connectors with out any effect.
Since you have had things working it sounds to me like a bad cord or connection. I would borrow connections/cords that you know are working and try again. That's as far as I can help I'm afraid.. Good luck!
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Poor picture quality, black & white only