Why does FPS drop from 60 to 25 during MGSV stealth missions?

I've had enough of ITX thermals; my frame rate would suddenly crash from 60 to 25 as the CPU throttled from 3.6GHz to 0.8GHz. I tried adding a small fan blowing directly on the VRM, but while temps dropped by 8℃, the drops still occurred randomly. I used the GamePP Fan Curve Configuration Panel to lower the trigger temp to 35℃ and locked speed at 2200 RPM. AIDA64 showed VRM temps held at 82-88℃, and HWiNFO measured CPU temps at 74-80℃.