Game in slow motion using directX

[-Drv-]

I usually played regnum using shader model 2, directx9, with low grafic effects (only particle effect active) on my pc but lately i can only play using opengl because when using directx the game seems running in slow motion.
http://www.youtube.com/watch?v=uqPGgkCyu80
OS: Windows 7 Ultimate 32-bit SP1
CPU: Intel Core 2 Duo E6300 @ 1.86GHz
GPU: Nvidia 8400gs
RAM: 3Gb

This problem appeared in the last month, before i had no problem using directx.

I also use the same configuration (Shader Model 2, Directx, Windows Seven) on a better pc without problems.