Why does Destiny 2 FPS fluctuate on Thermalright PA140?

During intense raids, my frame rate looked like an EKG, jumping between 50 and 80 FPS, which was incredibly frustrating. I tried lowering the CPU voltage offset in BIOS; while temps dropped by 4℃, the system started crashing randomly during scene loads per Event Viewer. I switched to the GamePP Fan Curve Configuration to force fans to 90% speed once the CPU hits 75℃. Frame time converged to 14-19ms per RTSS, and CPU temps stabilized at 72-80℃ per AIDA64 with fans at 2100 RPM per SpeedFan.