[drh4683]

That combo is a factory cabinet, 1966 model. Looks like an AGC problem. Check the setting of the AGC control and check the 6BA11 sync/agc tube. Also check the IF tubes. Rock the IF tubes around in their sockets to see if any snow shows up. Those sockets zenith used in the IF can sometimes have intermittent contact with the pins due to corrosion regardless of how clean the set is.
Im into checking all tubes even if the set works when I first find it. In this case, since the set has problems, thats number one start, then i look for missing or incorrect voltages if tubes turn out to be good in the suspect circuits.