1959 Zenith audio issue

[Olorin67]

Intermittent grid connection sounds likely, could also be another tube upstream of the audio output. Try cleaning the tube pins and sockets first, the look for poor solder joints. A grid can develop quite a potential by picking up electrons. Sometimes it build up enough to discharge, then the audio will come back. You can also test by momentarily grounding the grid when the audio fades, it will then come back. I had that issue on a 30s radio with the grid cap type tubes, touching the grid cap brought back the audio. Turned out to be a cold solder joint.