I've also had issues with the NIC on this motherboard however I was running UEFI. I had to try and get Secure Boot to reload all keys before the machine would boot with an active NIC.
For a short time, if I cold booted, the NIC would not be present. I had to warm-boot and on the 2nd boot, the NIC would appear and be available. It was a pain to have to do this workaround until I found out about Secure boot.
Problem I have to day is that I configured Hyper-V on Win8.1 Update and now I have the same issue again with have to boot twice. I have the latest NIC drivers and BIOS update so I don't think this is the issue.
This motherboard is temperamental!