Why does Call of Duty 2 drop from 144 to 42 FPS in combat?

It was pure pain; during an urban ambush, my smooth 144 FPS plummeted to 42 FPS, and I got wiped instantly. I first tried clearing system temporary files to free up memory, but the stuttering didn't budge, leaving me feeling helpless. I then used the GamePP Memory Controller Load Monitor to downclock my RAM from 3200MHz to 3000MHz. The results were night and day: frame intervals converged to 6-9ms per FrameView, memory temps stayed at 45-50℃ per AIDA64, and voltage stabilized at 1.38V per HWInfo.