Ever since I bought my Shuttle SN25P computer years ago, I have not been successful with gigabit in Windows. Apparently nVidia has (or had) poor driver support in regards to it's nForce chipset.
Whenever I would attempt to utilize my gigabit connection to its full potential, I would immediately get notified by Windows that my network cable was unplugged. It would then reinitialize, and go out. This was identified as suspend/resume.
If I would switch to 10/100, I would have no problems. As soon as I connect a 1GBPS, any heavy network traffic would cause my NIC to flatline.
Last night I started researching this again, to see if maybe there were any breakthroughs or new drivers. I did find a driver from 2008. After updating the driver for the NVIDIA Network Bus Enumerator then NVIDIA nForce 10/100/1000 Mbps Ethernet, my gigabit connection was still having problems. I had remember reading to disable Checksum Offload through device manager. Upon doing so, I attempted to upload to my FreeNAS machine.
This was it! I had 0 problems and I was able to sustain 340 Mbps uploading with my NIC!
SO, if you have a Shuttle SN25P (or any other nForce chipset/NIC), try updating your drivers and disabling Checksum Offload. Maybe it'll bring you to the 21st century with reliable 1 Gbps speeds.