I am using Windows Network Card sensor to monitor NICs. I found PRTG allways show "down" status for NIC WMI sensor for some servers while large network traffic. Actually, the traffic is just 1/3 of max. of NIC's full load. However, other sensors (CPU, Memory, etc.) on that server are showing normal.
Any setting can make it better and at least not show "down" status since it causing some fault alarm.