I looked at your video again, and your TV definitely doesn't like anything over 240p/480i. It cuts the bottom off anything over that, even 248p. It is clearly struggling to even sync with anything over 240p/480i (see the zigzag at top). Unsurprisingly, it seems to be an NTSC TV (says so right there), so use the NTSC monitor preset as your template.
If you've used the NTSC monitor preset to regenerate video modes properly, GM should not be selecting modes with vertical lines over 240p/480i.
An in-game log could help. That starting info screen can help too, as it provides video mode selection and screen sizing information. You can opt to hide them, but I prefer to leave them there.
Another matter is the suspected vertical collapse. Whether that line at top is foldover or not, I can't be 100% sure without a closer look, but the image definitely seems shrunken and now I notice it seems to have bit of pincushioning at the corners. It only gets worse over time, picture shrinking from top and bottom. Final stage is a single fat bright line across the middle. Replacing old caps may help and may help prevent other damage down the track.