Isn't the picture showing ringing symptoms? These are tricky to resolve sometimes. I like to start by completely removing the video to carefully examine a blank raster for uniform luminance. The reason for this is that sometimes periodic luminance variations can get into the video signal -- sort of like barkhausen. If the the blank raster displays the problem then it's a ringing issue in the horizontal sweep stage. If that's the case I'd start with a fresh set of tubes to rule out any quirky stuff like "pinked" grids, neutralization problems, cathode leakage etc. If the ringing is still present after a fresh set of known working tubes then it's either a supply voltage issue or a bad component. Start with checking supply voltages. You may need an oscilloscope to verify the DC voltages are "clean" - no hidden noise. Make sure the scope has a timebase that has variable delay so you can "step" through each scanned line in the picture. Barring any of this being the culprit ... reflecting on the problem reminds me that ringing simply means the energy of the flyback is not being released uniformily during the 15.3 us sweep interval (time it takes the electron beam to move from the left side of the picture tube to the right). So, it could be a non-uniform drive waveform or it may be an impedance mismatch between flyback and yoke. Well ....there are many possibilities. That's the nice part about trouble shooting old color TVs. It makes you think a bit.
