Winking Zenith Porthole

Chassis 22H20 (H2227R). Works fine EXCEPT video flashes on and
off at about 1 second intervals. Tubes check out fine, have full screen with some retrace so I figure a cap is charging up and dumping its voltage; an electrolytic, perhaps?
Outside of trying to meter these voltage changes with the set
on, any suggestions as to where to start?

Thanks!