I have had the same problem on a new Nexus 8. All gears work well except 6. It shifts into 6 and then after a few pedals gives a terrible grind and the pedals pop forwards. This continues every few seconds in that gear. I have lined up my yellow lines perfectly before and during every ride, even going through all gears and back to 4 for the adjustment. Are there any other adjustments to make on this hub? What are the other colored dots for? Will those change anything?