Certainly could be a funky chainline.
Also, the fg crowd is well-aware that cogs/rings/chains sometimes take a bit to wear in together. With a spring-loaded rear derailleur or tensioner, this grittiness seems to be soaked up by the spring. Without the spring, you feel every little bump down there.
jim
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever