"System Interrupts" high CPU usage when copying over network

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http://hardforum.com – I have a Windows Server 2012 machine that hosts all of my files. When I try to copy files from a share on my Windows 8.1 desktop (via gigabit ethernet), the transfer starts fine (50+ MB/s) then quickly drops to 7-8 MB/s. Looking at the server's Task Manager, CPU usage has jumped to 80+%, with 50% being used by System Interrupts and ~30% being used by System. SMB 3.0 is presumably being used but encryption is disabled so it's not that. Nothing else is using the disk being copied from according to Resource Monitor. The strange thing is that sometimes it works fine, but as soon as I notice the (Hardware)