Strange colours when using supergun

[Arch]

I am trying out my recently aquired supergun, a Vogatek MKV with s-video output. I have set it up like a blueprint of the picture you see underneath (except for s-video instead of RGB out), JAMMA psu and all, and it seems to work fine. The only thing is that the screen goes a bit weird once the games start playing.


On my Rainbow Islands, the screen looks as follows:


The image is crisp and clear and most of the colours displays correctly but the background (which is supposed to be black in this game) is instead a full orange colour. On top of the screen, the colours are different. The border between the two moves up and down a bit and to me this looks like a meter of sorts. Is the pcb or supergun trying to tell me something, given that this also happens with other games, or is something borked? The composite out is the same by the way. I am totally new to handling pcbs, so this might have a "duh" answer. So feel free to "duh" me all you want please.

Cheers,
Arch

[Korszca]

It would help to see your actual setup if you could.
I'd also try the boards on a different supergun or a cab to make sure it's actually the supergun that's the issue.