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A couple of cases I've seen over the years:
1. If a wire with high current AC, such as a filament supply wire, runs close to the neck or funnel of the tube, this can happen. The magnetic field from the wire is enough to cause this kind of problem. So check wire routing of AC line and filament supply wires and keep them away from any unshielded part of the CRT.
2. If you replace a selenium rectifier with a diode, there can be crossover current spikes when the diode switches. These can get into various things and cause a small "tiddle" on the picture that travels slowly up the screen. From your description, I don't think this is it, since you describe yours more like a rolling barrel distortion. In any case, when you see this problem, adding snubber caps across the diodes often helps.
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