Why is my FPS unstable in Unknown 9: Awakening at 4K?

While sneaking through buildings, the frame rate jumped randomly between 60-90 FPS, which was very obvious at 4K. I first tried locking the max frequency in the driver; while FPS stabilized, power consumption soared and fan noise became deafening. I used the GamePP Fan Curve Configuration Panel to set the trigger to 60°C and full speed at 75°C. RTSS showed frame times converging from 11-30ms to 8-14ms, and HWInfo confirmed GPU temperatures stabilized at 65-72°C.