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Thanks for the suggestions everyone, I dug back in today and here's what I've tried and the results.
Subbed filter and doubler capacitors: N.b., the B+ was fine before, in fact maybe 20V high. Swapping in the new filters and doubler made zero difference at all: B+ didn't vary more than a few mV.
old_tv_nut gave me an idea, I noticed the degauss thermistor had been replaced by some weird-looking thing I've never seen before. It measures the same cold as the original piece, and anyway I removed it from the circuit to be sure I'm not seeing magnetic effects from the degauss circuit. No difference at all.
I decided to tackle the vertical circuit next. Voltages on the tube are correct or a small amount high, output cathode is at 19.86, osc plate is at 210, output grid is at -18.2. None of the voltages vary even a tiny bit. The waveform on the grid of the VO looks exactly like the picture in the Sams, measures 211V P-P, and is completely stable, no change at all in amplitude or frequency. I tried to get a photo but at that slow sweep rate the camera wasn't cooperating.
Tell me if my thought process from here makes sense: it looks like the vertical circuit is operating as it should. What we're seeing on screen is pretty significant; if it's affecting the vertical it should be immediately evident in any of the waveforms or voltage measurements. Since this issue is affecting both the H and V equally, and in sync, and the B+ is both correct and stable, I'm thinking it could be a problem with high voltage. If the HV were fluctuating rapidly, it could cause exactly this symptom, no? The Boost voltage seems stable. The set was hissing a bit when I first brought it up but I blew out the HV cage and it stopped. I probed the HV and got 17kV. It didn't seem to be varying, but I don't know how accurate these meters are or how much variation would cause our symptom.
So that's where we are. This is my first color set. I'm suspicious of the HV but not sure where to go from here. Any other ideas?
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