Heathkit GR-53A color TV

[DaveWM]

I still think you should check the grounds, if there is a common ground for the deflection board (assuming it has a PCB) then a defect in the ground could see a beat freq with the vert vs AC , then share that with the horz (same ground on pcb) and get some modulation on the horz tube cathode. Power up the filaments on the deflection with a 6v battery and see if it goes away. On my hula it would take something like 10-15 seconds to work its way thru (a vert line would show a slow moving horz shift like a hum bar, but much slower).