Ooo, sum ting not right for sure. I'm picking up almost a rectified sound, the 'octave-like' tone. Then, of course something happens that dumps the volume.
A couple of things come to mind...something strange with the bias...maybe a polarized electro cap in the signal path backwards...a missing pulldown resistor somewhere....bad bias in the power section (?) causing that cutout (R values...). Lack of grid stoppers maybe. Does it cut out like that if left alone (not playing; no input)? Does it come back in a bit if you stop playing?
- can you affect this dropout by using the eraser end of a pencil as formerly mentioned, to move wires/sockets a bit, to see if something is loose?
If nothing seems loose, I'd try to get it to happen again, and audio probe through the preamp (triode) signal path from last preamp stage back toward input, see if there is a spot the sound seems normal while it's cutting out.
BE CAREFUL, respect the voltages where you are probing, and be sure everything you touch is fully insulated and the cap in the probe is of higher working voltage than the B+. It's helpful to have something inputting a 'noise' while you probe so YOU don't have to....a test oscillator or even tunes from a cell phone or something. It's awkward to try to pluck strings while you probe, esp. with high voltage there...