if at all possible I would tap into the 6592 directly and see if you get any thing there, if not then you'll rule out trying to find something between there and the plug.

also isn't the 6592 the one labeled S-ENC some times? you could see if you get component video off of it if its the same as the S-ENC's and maybe find if a particular channel has gone bad (y, pb, pr) or something