If the previous extreme measures don't help, then it may be possible to simply substitute a good divider network externally.
Note that if there is evidence of internal breakdown in the divider of the original flyback (hissing, cracks, overheating, bulging case, etc.), this will not work unless you can disconnect it from its HV connection.
There are two issues:
Various size external focus/screen divider networks can be purchased but whether this is truly a cost effective solution is not obvious.
(From: Larry Sabo (firstname.lastname@example.org).)
I just ordered a 'bleeder resistor' from Data Display Ltd (Canadian sub of CCS) to use as a cure for flybacks with flaky focus/screen pots. It contains focus and screen pots, and costs Cdn$ 16.99, which is a lot less than a complete flyback, that's for sure. I expect it will be compatible with quite a wide range of flybacks.
I have used bleeder resistor assemblies from duff flybacks a couple of times with good success. You connect the HV lead into the HV cap of the original flyback, ground all pins of the sub flyback, and use the focus and screen leads from the sub bleeder assembly in place of the originals.
Looks like hell but works fine. Mounting (and securing) the substitute is a challenge given the limited space available. I only use this approach on what would otherwise be uneconomical to repair, and always advise the owner or customer of the cobbling job. It also enables you to verify whether it is the flyback that needs replacement, versus the CRT.