Why does frame fluctuation persist after lowering graphics quality?

Realistic shadows looked great, but frame rates dipped unpredictably on the plain. Lowering quality sacrificed too much visual detail. I used GamePP BIOS Load Line Voltage Adjustment Wizard to set voltage to 1.28V. CPU temp stabilized at 70-75℃ per GamePP Hardware Sensor Page. Frame rate held above 60 FPS per In-game FPS Counter.