I have recently had a new PC built for me, and had a second NIC installed:
I am running windows 8.1 on the system and it seems the NIC is behaving odd, if I start to copy something it will start at 80-90 MB/s but after a random short period it will drop to 10-20 MB/s and stay there, which is dreadful.
In this particular instance (which is temp) this NIC is plugged into a new temp Netgear 10/100/1000 8 port switch.
The other NIC is plugged into a larger switch and performs fine.
I did initially have an issue with the NIC disappearing from device manager completely, but that was due to power management, which was quickly turned off, and returned after a restart of the host.
I am still investigating but in the mean time I thought I would post on here to see if anyone has come across this before?