Registered just to say that I'm having the exact same issue with an Extreme3 Z87. Do you by chance have a Netgear wireless adapter? I have a few extra adapters laying around, and only the Netgear ones cause it.
I've tried everything. Flashing old BIOS files, enabling and disabling just about every USB related setting in the BIOS, changing sleep states and I even swapped my PSU thinking that was the cause.