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Have you tried swapping the video stage tubes and deflection stage tubes. If there is heater-cathode leakage that could cause it and or other strange symptoms.
It might also be worthwhile to feed it a blank raster and check video and deflection waveforms at the CRT. Video should be just sync pulses if you're feeding it a blank raster and deflection should be a clean sawtooth. If you see anything else especially if it is a short pulse then you have found the line the noise is on (then comes figuring out what on the line is causing it).
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