what could be the problem on my p5w dh

whenever i shutdown it always restart

things ive tried
* bios upgrade
* bios downgrade
* cmos reset
* disable power on by keyboard,mouse, lan etc

ive remove any usb device on the usb 3&4 "the one for the usb remote" and now it doesnt restart after shutdown

its just happen just now but my setup is working fine before for almost 1yr

+ used multitester on the usb port and ive found out when its on standby mode it only emits 2-3v and when i turn it on it goes to 4.5-5v which is normaly before even on standby it should give me a 5v

so does my usb port is busted?