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Your vertical osc. is running off frequency: primary causes bad and or drifted value vertical stage resistors, caps, tubes. Check and replace as necessary.
Also since spot persistence varies you may want to check all B+ voltages in the vertical stage, especially any shared with the CRT electron gun and horiz/HV system (since the change in spot time may indicate a change in PS loading/operation).
Don't worry about spot time too much. The spot was something later sets designed out of the system.
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