I can reproduce this on 60hz monitor - for me fluctuations happen 30/60 fps
Can be mitigated via env variables:
KWIN_TRIPLE_BUFFER=0
KWIN_X11_NO_SYNC_TO_VBLANK=1
KWIN_X11_FORCE_SOFTWARE_VSYNC=1
KWIN_X11_REFRESH_RATE=60000
here 60000 is for 60hz, OP would need 120000 probably