Tom, the only way to find out what is causing this behavior is disconnecting all peripherals and then start adding one at the time in order to find which one is causing the conflict.
Please test the system out of the chassis in order to avoid any grounding coming from the chassis.
If you have any USB 3.0 driver installed, please remove it. Then, restart the system and install the following driver. I know you are using Windows 8.1, but please give it a try.
