I put the HV supply back in its enclosure to observe behavior... no better, HV continues to come and go. When it is working ok, neon indicators show voltage on all coils:
When it quits, the neon indicators next to the higher voltage coils do not light:
Suspecting that the problem might be in the highest coil, I removed it and built a simple voltage doubler to recover the lost voltage. This approach seems to be working, as I can easily get 5.5 kV or more, with only about 10 mA being drawn by the 6C4. I have to be careful in adjusting the trimmer cap, to not exceed the 6kV rating of the multiplier caps.
Perhaps there is hope yet for the old Hallicrafters!
jr