How to fix Returnal FPS jumping between 40 and 90 FPS?

Facing enemy hordes, my frame rate jumped wildly between 90 and 40 FPS, which felt jarring. I tried lowering all graphics settings; while average FPS went up, the erratic drops remained, making it a superficial fix. I then used the GamePP Power Plan Switcher to enable Ultimate Performance and disabled core parking. GamePP showed core load at 99% during peaks. The GamePP Frame Time Analyzer eventually confirmed a fluctuation rate below 5%, with frame times stabilizing at 5.1-6.4ms and temps at 78-84℃ per AIDA64.