Just done some more checking for you and it appears to be a fault with the NIC. Basically the NIC number, stored in a "secret" part of the firmware was not programmed correctly or a bug corrupts it -- whatever. It's gone now and upgrading the BIOS or erasing the CMOS etc doesn't help.
So after more searching around it appears that there is a NIC address flash setting utility that can be used to sort it. Im having a look for it now for you. What i would suggest is that you get in touch with Asrock and ask tech support for the utility to fix the issue, incase i cant find it for you.
This is apparantly the tool you need, its for another board, but it cant hurt to try as people are having success with other models
If that doesnt work then take a look at the last help question on the page for MAC addresses aswell.