Why is my frame rate jumping between 60-90 FPS in shadow scenes?

While sneaking through the shadows, I noticed the frame rate jumping randomly between 60-90 FPS, which made me very uneasy. I tried disabling all unnecessary overlays in Device Manager, but this caused severe drops in my livestream. I used the GamePP Peripheral Driver Conflict Detection Tool to locate conflicting addresses and switched the service state to Isolated. RivaTuner confirmed the frame interval converged to 8-12ms from 12-45ms, and GPU-Z showed a stable 58-64℃.