[old_tv_nut]

The noise inverter could affect sync, but I would expect it to affect both vertical and horizontal.

The purpose of the noise inverter is to invert RF noise pulses that go beyond the sync tips, quickly clipping them and making them go towards white instead of super-black. This shortens pulses like ignition interference greatly, so that they do not overwhelm the sync separator. What would be a wide super black pulse gets turned into a narrow super black pulse followed by a wide white-going pulse (I mean that it goes in the direction of white, not that it necessarily goes all the way to white or even all the way to black level). The following RC filtering before the sync separator then tends to reduce the negative part even more (because it is now narrow), so the sync separator is much less likely to react to it.

Edit: The noise inverter could be degrading both H and V, but if one of these is more sensitive to the degradation, it could fail separately. So still worth checking the noise inverter even if only vertical is a problem.