I was having the same problem. i was fortuned to have 2 usb front ports that belong to the case, and work just perfect!
Now the problem is semi-fixed because the problem with the onboard usb ports still exists!!
By the way, i have the same motherboard P8Z68-vpro/gen3 and i7 2600k processor so the question is what does this mean???
Any clue??
Best regards