Try clearing the CMOS by pulling the battery with the system unplugged and count to 10. Set the BIOS defaults and reboot. Go back into the BIOS and config as necessary.
This problem has happened to me twice already. The first instance was fixed when I gave in and added a second Intel NIC to the box. I presume that jolted the BIOS enough to remedy whatever corruption was there and effectively cleared the problem. Try my first suggestion first though, that should remedy the problem for you...worked for me anyway.
- ShY5TR -