Had a similar problem. I too thought the chain guard was properly adjusted. I took the chainguard off and the problem went away.
If you are getting good gears, I would look somewhere other than the hub.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1