need to stable clock speed

hi
i m running my comp on an amd64 x2 3800+ and asus m2nmx motherboard with 2gb ddr2 667 ram and a nvidia 7300gt card. when i first bought my computer, i once ran prince of persia sands of time and it worked perfectly. now when i install it, it runs wierd. the character speeds up suddenly and speeds down suddenly. i believe that this is due to some power management thing of my comp that adjusts the cpu clck speed. if so , how can i disable it. i have also tried turning vsync on but that doesnt help either. please help