Solution!!!
I changed the Intel X520 NIC for a Broadcom 57711 NIC and this it's working excellent!
In Windows 2012 RC:
"Intel 0 - Broadcom 1"
You might want to try the first software release that includes drivers for Windows Server 2012 at http://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=21642. (These drivers weren't here last week when you first posted.) Just ignore that the OS says Windows 8 instead of Windows Server 2012. The OS hasn't been added to the list yet. That addition is coming soon.
If you do try out the new drivers, let me know if that fixes your issue.
Mark H
Thanks Mark! I'll try this driver and I let you know the results.
Hi Mark.
I installed the new drivers and they are working very well! The installation was very easily and I can see some new features and options. Good job Intel!!!
Again thank for your response.
In Windows 2012 RC:
"Intel 1 - Broadcom 1" 
