Based on the tests you've done, I'd recommend the following:
1. Just to be 100% sure, restart the system into BIOS by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system. Test.
2. If the problem continues, test on any of the USB ports on the back panel of the board.