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.
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.