A few weeks ago, i had some really weird issues, when i was using two NIC cards on my Arch install.
I was having SLOW network speeds like 30/40 network transfers etc.
As soon as i removed my USB to Ethernet adapter.. It started working flawlessly. I still haven't' figured it out yet.
Is the network card that's having issues onboard NIC or a network card you added? It may have bad driver support.