[old_tv_nut]

The problem is most likely in the antenna leads, but I suppose the signal from your source could be weak - do you have another good TV to try it on as a quick sanity check?

The "big noise" is normal switching transient when you change channels. Even though the sync separator circuit may see it as a momentary erratic sync signal, it should not cause the vertical to collapse. This would lead me to check components between the sync separator and vertical sweep section, as well as components in the vertical section itself.

Thumbs up on the audio noise!