[Eliot Ness]

When this happens if you just turn the volume control a hair one way or the other does it go back to normal?  That volume control (if original) is about 90 years old now so it might need an occasional, or maybe initial cleaning.

You might also want to clean the pins on your tubes (a very fine emery cloth works well for that).  Maybe lightly tap on various tubes with a non-metallic object (wooden chopstick maybe) when this is happening.  You might want to substitute in a different set of known good tubes. 

I also like to check and make sure the tube sockets are all tight and then I clean them with some contact cleaner on these little brushes I got from Harbor Freight for airbrushes:

https://www.harborfreight.com/airbrush-c...68155.html

   

I'd also suspect any original resistors that you didn't replace and also look for any cold or suspect solder joints, both old and new.  An intermittent problem that only happens for a few seconds can be hard to find.