1959 Zenith audio issue

[bigaudioal]

Anyone know what might be causing this condition with my 1959 Zenith? Set has been fully recapped. Tubes test fine and swapped audio tubes as well. Out of tolerance resistors all replaced too.

When I turn the set on it is fine for the first 5 minutes. Runs beautifully. Then all of a sudden, the audio fades out slowly to nothing.

If I leave the set alone, after a few minutes the audio will come back to normal all by itself. Just pops back in. Does not fade back in. I sometimes can make the audio come back faster by changing the channel. Only does it once each time I turn it on. After that one time, I can leave the set on the rest of the day and it is fine. Any ideas???