Look closely at the blanking. It almost looks like the scan lines are getting folded together.....for lack of better phraseology.....

So there are little "step function" jumps in the vertical position as the tube is scanned? Noisy resistors? Dirty vertical position pot?
jr

I replaced almost all of the resistors in the vertical section, but there were a couple that I had to rob out of the parts chassis because I didn't have suitable replacements. Hm.

Other than testing the resistor to see if it's in spec, how does one spot a "noisy" resistor?

That description almost makes me think line pairing. Does it look like every other scan line is missing when this happens? I can't really detect what you're describing from watching the video.

Since the problem occurs after the set is "warmed up", perhaps blasting suspect components with "circuit cool" might reveal the source of the problem.
jr

Often some judicious poking and prodding (with an insulated tool), tapping tubes (including the CRT neck) will reveal an intermittent component.
Freez Spray ("circuit cooler") can be useful if the condition is heat related.

The set's now in the hands of my friend Al, in MD. It's generally been performing well for him, other than that intermittent raster issue.

Since Al is pretty handy and not a stranger to the soldering iron (and loves this stuff), I suggested that he replace the remaining resistors in the sweep circuits.

I probably would have, but ran out of time......he was in to visit family for Christmas :)

Resistors ordered.

ID, Snip, solder. Repeat! :D