Sometimes there is network slowness (connected 1GB) while transferring files in network from one server to Dell R710. The transfer drops from 100-90 Mbps to 6-15Mbps and after sometime recovers itself. Updated the Ethernet drivers still it isn't resolved the issue.
How did you come to the conclusion that it is an issue with the NIC? Based on the information you provided it does not sound like a NIC issue. It sounds like a drive read/write issue on either the sending or receiving server.
I suggest that you perform more information gathering. I would test with the exact same file and transfer it from the source to multiple destination servers to see if the slow down is occurring on all servers. If the slow down is only occurring on this server then test the drive read/write speeds.
Dell EMC, Enterprise Engineer
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To rule out the r710, off hours connect another server with a cat5 cable directly to the r710, test. Turn off flow control if you have a relatively small network, disable IP6 on the NICs.