Bumping this topic up, because I am facing the exact same problem in our edit suite. Mac Pro Late 2013 connecting to a Windows 2012 R2 server over 10GbE with the Sonnet Twin 10G TBL2 Adapter. Speeds are great but will randomly drop to miserable lows after a short time. Lower than 1G even.
Only thing to solve this is to restart the Sonnet adapter, what will bring the connection back up to speed. It will however lose speed pretty much after a couple of minutes again. Very frustrating, especially when you are not a networking pro…