What's causing this ramp on a video signal?
I'm guessing that the source is AC coupled ( and shouldn't be)

you've got dc offset shifting with picture brightness. Video should be DC coupled and terminated into 75ohms. If that sounds like something that your wouldn't string across a room, congratulations; it was never designed to. I remember reading that in-studio distribution was never raw analog video. So check the source path for a cap.

Unless it's just a really bad ground connection (high resistance) ;and the ground is elevating with brightness, which is consistent with that. So check the return path resistance too.

Analog video; it's frustrating because it was never intended to go very far as a baseband signal.