The RCA docs are here.
http://www.antiqueradio.org/art/RCA630TSService.pdf

What it sort of sounds like, that the -100v source it changing drastically when you make those adjustments, and it's messing with the Horz sync, have you checked that it is stable when you adjust focus, contrast and brightness?
OR the other bias voltages when you adjust them?

Yeah looks like I have some more work to do with the multimeter and scope. Oh well, I solved one problem…I’ve got a picture stable in the vertical. I’m gonna run this down whatever it takes!

Just a shot in the dark, and a long one at that -- does the jug have a dag coating, and if so, could the dag have a flaky ground, or a floating dag "island"? I've seen a few weird effects that first appear sync-related but traced back to flaky dag.

I think he said he's using a 5AXP4 for testing, I used a 8YP4 when I worked on mine, It had no Dag, and it did not cause any issues.

Yes it’s a 5AXP4. It doesn’t have a dag coating and I don’t think it ever did. Ive used it successfully in lots of early black and white 📺s

Sounds like this tv is one of those spooky mysterious flaky TVs where resonances and rogue RF signals wreak havoc on performance. I’ve always have had trouble with these “Ghost in the Machine” sets.

True, I have found it to work better on chan 7 than 3.
My FADA 895 has been giving me very little trouble , just the odd annoying once in a while.
The horizontal sync has been extremely stable, rarely have to touch the adjustment on the back, only when really cold weather hits.

Only when cold weather hits? God help me!

Yes, sometimes summer vs winter, when it's really cold, the horizontal hold control on the front won't bring it in sync, and have to make a VERY slight turn horizontal freq adjustment on the back, like 1/16th inch of a turn, if even that.

Now what’s that all about? How does it being cold outside make a difference when your house is the same temperature as it is in the summer?

Not quite, I have it set up with the back facing the north wall, and it summer, AC is set to about 77/78f and in winter, heat is set to 65f. :P