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I have Shimano Nexus 8-speed internal gears on my bike. They have a subtle problem that has been getting worse over time. Any time the pedals are turning with some resistance on a hill, the gears keep making a clunking sound every 4 or 5 seconds and the pedal jolts forward, as if the bike cannot settle on the desired gear. This happens even on gentle slopes and on the lowest gear.

I have attempted to recalibrate the gears using the cable knob on the handlebars and some online tutorials. Specifically, when I turn the bike upside down, there are two yellow marks that I align by adjusting the cable.

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Upon aligning the yellow marks, gear shifting improved somewhat, but the unstable clunking got worse.

Is there some other calibration I need to perform? I also see some red and yellow dots behind the frame.

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If you have the yellow ticks lined up like that when the shifter is in gear 4, that is correct and you've done all you can in terms of normal adjustment. The other dots are for lining up the cassette joint properly prior to installation, and you otherwise ignore them.

The next step from where you're at is inspection of the hub internals and oil bath service. Shimano recommends this is done 1000km from new and every 2000km or annually after that. The pedals jumping forward under pressure suggests an issue with the pawl system that allows the core assembly to drive the hub shell. Once it's apart, this is an easy to inspect part of the internal assembly since it's right there on the outside, and simple lubrication may clear it up if things aren't too damaged.

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