hi, i am asus supporter, but i am facing a problem when playing dota 2, can someone tell me what is my pc problem, or which part not suitable for my pc

window xp
mother board asus M5A78L-M LX V2
graphic asus ATI 6670 2G
4G DDR 3
500 hardisk

when start playing dota 2, FPS is normal 50-60
on half way playing game sometime will lag or jam half or 1 second, when time jam the FPS will drop to below 30FPS,
after 1 second the game will continue as normal, FPS back to
normal 50-60.

i have try and change my graphic to asus GT 640 and processor change to FX4130,after testing the
result is the same, will lag sometime.
don't know y FPS can drop suddenly.

anyone know this problem ?
or got other friend using this motherboard play dota 2 happen this problem also ?
Or this motherboard got bottle neck for dota 2, can lag sometime ?