Weird usb problem

Hey i'm having some werid problem...

I have a belkin bgn wifi usb adapter plugged in the back of my motherboard and i get about 25 Mbps, I also have a maudio fast track mkII usb sound card, the problem comes when i plug it in ( wifi usb 2.0, fastrack usb 3.0, i've tried diferent combinations) then my internet speed goes all the way down to 4-6 Mbps.

Don' know what's happening.. need help! thanks!

System:
i5 3570k
asrock z75 pro3
8gb corsair vengance
asus gtx 550 ti