Asus p5kpl-am in motherboard doesn't power on

Hello,
when power supply is given, green led glows constantly. if power button is pressed cpu fan runs for few seconds and stops then starts again and pc runs ok. if restart is made it shuts down and doesn't power on.

i dismounted memory, processor, harddisk, optical drive; cleared bios, jumpers changed but the cpu fan sometimes runs ok, but sometimes it's completely dead.

i tested smps with other motherboard and it's ok. i think the motherboard may be faulty.
how can i resolve it? is there any problem in power on-off ic?