Yes, in further puttering, I returned R55 and C44 (Sams numbers) back to the values given in the Philco docs. I had tried different values for them at various times, including an alternate Sams value. I also swapped amongst my three 6DR7 tubes until I found which one my set likes best. Linearity and vertical lock are both improved.
Then I ran through the horizontal & audio alignment procedures and played the set for a few hours. Nice and stable.
Those pics are from a DVD player, which has a strong signal and tends to swamp some sets. When I switched to the built-in rod antenna, it worked very nicely, receiving the relatively weak signal from my Blonder Tongue modulator on the opposite side of the house.
Not perfect, but I can see light at the end of the tunnel.
Phil Nelson