Why does my GPU frequency drop when racing in Ghost Manor?

Racing in Banishers, my GPU dropped from 2400MHz to 2100MHz. I added case fans, but it barely helped. I used GamePP Hardware Sensor Page to tune the fan curve. GPU-Z showed frequency stable at 2350-2400MHz with temps at 72-75℃. GamePP Benchmark Verification confirmed the boost. I finally can verify the stability.