Why does FPS jump between 120 and 180 in DS2 firefights?

It's so annoying! Every time a firefight starts, the frame rate jumps wildly between 120 and 180, creating a stuttering mess. I tried lowering all in-game graphics settings; it only dropped the temp by 3℃ while destroying the visuals, which made me furious. I used the GamePP Power Plan Switcher to set the system to High Performance and locked game threads to high-performance cores. RTSS showed frame time jumps reduced from 8-35ms to 6-12ms, and HWiNFO confirmed CPU temps stabilized at 65-72℃.