How do you use S-Video?
Is it through SCART?
As some S-Video (Like SCART in most cases) are not Real S-Video, but simply Composite.
And using that will result in much crosstalk (As Luma and Chroma is separated in S-Video, but if it's Composite in it it will be Luma = Luma/Chroma, Chroma = Luma/Chroma. And you can guess the mess.
Usually you get like a checkboard pattern with colored dots that falls down, yours is much more extreme, i am guessing it's some grounding issues.
Still all just a guess
