Why is there a power loss when a motorcycle is climbing a mountainous road?

My motorcycle, Honda CG125 (single cylinder) is in perfect order… Runs fine on leveled road.

I have noticed that while climbing a mountainous road, initially it would run fine at lower gear and high rpm (7000+). But after some time, about 30 minutes, it would loose it power. Quite interestingly, I have noticed same behavior with other bikes.

I would like to have some technical reply i.e. How the power is lost at high RPM under load.

Shall appreciate, if links to some technical article and/or some animation is provided.

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