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could be AGC acting up. does the contrast adj work ok? try wiggling the AGC tubes, move the agc adj pot back and forth a bit.
If it was just a horz issue I would guess the AFC diodes, but if is seems like the pic fade is with the sync lock, the AGC is suspect.
If you have a variable dc source you could try clamping the AGC and see if that settles it down, check the 4uf cap in the AGC circuit.
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