Why does my GPU temp spike and drop FPS in Far Cry jungle scenes?

While sneaking through the tropical rainforest, I was baffled by how quickly my GPU hit 82-88℃ per AIDA64, causing frames to plummet from 90 to 45. I tried opening the chassis side panel, but while the temperature dropped by 4℃, dust accumulated rapidly and noise became unbearable. I then used the GamePP Fan Curve Configuration Panel to force the front fan speed from 1100 to 1600 RPM at 60℃. After this, GPU core temperature converged to 72-76℃ per GPU-Z monitoring, and fan power fluctuation stayed at 2-4W per HWiNFO.