Need some help with a baffling networking issue

Server A and Server B are identical, hardware-wise and configuration-wise. Both NFS mount several shares from Server C.

Server A has developed an issue where it is 10x slower than Server B when writing to Server C. Also, Server A has the same transfer times/speed as server B when transferring to other servers on a different subnet.

Server A also has the same bad transfer rates with other servers on the same subnet as server C. So it seems Server A, and only Server A, has an issue with the subnet, and only that subnet. SCP behaves similarly.

Any thoughts?