Originally Posted by merztime
I have this same tight and loose spot on my Pista with all-stock drivetrain. Originally I thought the culprit was the rear wheel alignment, but that doesn't seem to be it. I also suspected the chainring, but I'm not convinced thats the problem either. I guess it could be the cranks or BB... For now I'm just dealing with it.
I would guess the chainring is out of round. Most are, though some much more than others.