If it was not for the rarity and value of the set*, if I found myself in your shoes I'd be tempted, after all that fruitless troubleshooting, to simply bridge the fuse and see what if anything lets go...
*Then again with all the shot CRTs and some loose chassis out there it may not be the worst set to look for parts for...
Sometimes the best way to figure out an intermittent is to stress a set till the issue becomes a near constant. I had an issue with a '71 Zenith hybrid where a color was dropping out and others were shifting bias from it...It could hold it for 15 min straight, but if I changed channels, or whacked it right it'd be fine...I could never get a meter on it. If I had it set up to take measurements it would just sit there and work properly for hours...Maddening! I kept it in daily use along side a Silvertone roundy (I like redundancy when using tube TVs) until it got so bad it was harder to make it work right than wrong....Then I was able to track which CRT element was not being driven right then trace out it's driver stage. All that seemed like it could be wrong was a transistor and swapping it with another color of it's video out stage confirmed the issue.
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