What would cause this file transfer slowdown?

This screenshot is of me transferring a large file (a .VHD) from one logical volume (made of 2 spindles in RAID0) to another one (made of 2 spindles in RAID0) on the same physical machine. (This is not production so don't worry about the sanity of that hardware configuration.) I initiated the transfer via explorer.exe on a third machine on the same LAN, so some TCP protocol may be implicated in the crime here; I don't know.

As the file transfer starts, it peaks out at around 220MB/sec, which is the speed I would expect from a volume to volume transfer on this machine, and then right about 50% of the way through, it drops down to around 50-75MB/s and finishes out at around that rate.

Any ideas as to why the file transfer rate from volume to volume consistently halves half way through the transfer? I tested the same transfer a few times and got the exact same behavior each time.

EDIT: I have tested this again by using robocopy instead of Explorer. I'm using a different file but I'm still initiating the copy from a third workstation:

I did not observe the drastic slowdown about halfway through the file copy using robocopy, however, if you look at the transfer speed at the end, it is still well above the theoretical limit of gigabit Ethernet.

EDIT #2: Here's the same transfer viewed through Explorer. There's no slowdown with this file. The only difference is that this VHD is about half the size of the first one:

Consistent evidence from two different tools that the file copy speed is faster than what GigE should be able to provide, so I'm still not convinced that the transfer is going over the network. But I still have no idea why the bigger file in the beginning would suffer from the slowdown halfway through the transfer. Perhaps there are two many variables/factors in this experiment.