Well, the doorknob cap was bad, after all. When I played the set to check another issue, a faint tick-tick returned in the HV section and then the cap failed, causing hash in the video and loud static in the audio. When I swapped in a doorknob borrowed from my National TV-7W (which has four to choose from), the picture stabilized.
I don't have a cap checker that can test at such high voltage, but out of curiosity I temporarily clipped the old doorknob cap in parallel with the good one. It immediately started hissing, so I guess it really is a goner.
There's more work to do on this set, but I'm counting this problem resolved. Thanks again for the good advice.
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html