Why is my FPS jumping between 40-70 in The Witcher 3 Next Gen?

Walking through Novigrad, I noticed the frame rate jumping violently between 40-70 FPS per RTSS. I first tried enabling performance mode in the drivers; while peak FPS rose, the minimums still hovered around 30 FPS per RTSS, signaling a memory bandwidth bottleneck. Using the GamePP Resource Scheduler, I found 20-35ms latency peaks. After a failed attempt to overclock to 3600MHz that caused a BSOD, I locked the mapping page size to 4KB. 3DMark tests showed frame times converged from 15-45ms to 12-20ms per RTSS, with AIDA64 showing temps at 42-48℃.