Well, I turned the set up on its side again to take some measurements and noticed that it was back to the Horiz-Vert bouncing again. I turned it back over to try to get video, and...it was only moving vertically. Horiz was solid but vert was bouncing like crazy at the top of the screen but hardly at all at the bottom. This HAS to be some kind of intermittent bad connection, I thought, and spent the entire afternoon poking, prodding, twisting, wiggling tubes and components, tapping all over the circuit board, moving wires all over the place to see if wire dress could be the root of the problem, and...nothing. It sat there with only the vertical problem for two hours. I turned it over a dozen times. I ran it upside down. Nothing!
I got an idea that maybe this was related to convergence or magnetism or who knows what else, and grounded the grid of the VO tube to take that out of the equation. I got a straight line, no large movement, but it's still jumping a tiny bit. Feeding in a crosshatch, I was able to clearly see that the pattern of horizontal movement is not present. In the attached video you can see the line moving slightly, which I suppose is a clue.
https://photos.app.goo.gl/WhGtJnhDhqmjUFdw9
So here's where I am:
I don't know why the problem has manifested in two different axes at different times. I went back this afternoon and messed with it for another half hour and it continues being only vertical. I am going to try to solve the vertical problem right now and worry about the other one if I see it again.
The fact that the vertical line exhibits a bit of movement at a similar 2-5Hz frequency, even though the vertical oscillator is disabled makes me think that...I'm not sure. That line pretty much can't move unless there is current flowing through the primary of the VO transformer.
Schematic
https://i.imgur.com/MVZOy8J.png
The only things that could cause that line to move without the vertical oscillator running are:
1. Maybe the convergence coils but looking at the schematic they are in series with the yoke so, I guess not.
2. Some stray magnetic field but I'm confident there is none.
3. Maybe the vertical output tube is still conducting current even though its grid is grounded. (Is this possible? I need to measure the voltage on the cathode). IF it is still conducting a bit and we still see our problem on the screen, then we can assume that the problem lies in the circuit between the 270V B+, which I believe is clean, the VO transformer, output tube, and that bypass capacitor. It can't be much else. Please point out any holes in my logic, I'm trying to learn.
My plan of attack for tomorrow is:
1. Watch the B+ line with an analog meter and/or a scope to see how clean it really is.
2. Check the voltage on the K of the VO tube; it should be 0 if not conducting and have some voltage if it is (lin control is set to middle of range). This would be a good place to check with scope as well.
3. Determine the defective component based on the above tests. The earlier problem in the H axis makes me suspect that 270V supply but I watched it for a little while today with a DMM and it only bounced around 20-30mV.
Barring that, well, I guess ghosts.