Thanks guys. I suspect the set is more sensitive to tubes because of the replacement flyback. If tubes hadn't helped, my next step was to try different values of cap in the yoke.
I got that idea from this excellent book "Horizontal Sweep Servicing Handbook" by Jack Darr. An entire book just on the horizontal sweep circuit. Heavy reading but very informative.
You can get a free, scanned copy here:
https://worldradiohistory.com/BOOKSH...-Darr-1963.pdf