[Penthode]

I see there mention of the little high voltage ceramic or mica capacitor across the left side of the horizontal deflection windings of the yoke. I believe it is to balance the distributed capacity but also I think it helps "tune" the left hand side yoke to prevent these ripples.

Typically it is valued between 47pF to roughly 120pF. I recall they were often selected on test and weren't even assigned an exact value.

You might try experimenting slightly raising and lowering this capacitor value within the range above to see if there is any improvement.

Come to think of it, is the capacitor present?

Also while you are at it, you might confirm the 560 ohm vertical winding damper resistors are still good.