the same issue here since the early days. Only thing to "fix"it is to disable the second nic. this did the trick for me but is not as it should be.
I want my already bought 2 nic back !
Its not an issue of cable or switch or whatever besides the motherboard.
Intel - FIX IT ! or should i change the board with the reseller ?
The issue has been reported and it is under investigation. The weird part is that not all of the systems experience this behavior.
Please try uninstalling your current driver and installing the following one:
Please note that there are installation instructions on the web site for the download.