Slow file copy across fat pipe

Hello -

I have a Windows 2003 server in Virginia, and another Windows 2003 server in
Los Angeles. Each has a public gigabit NIC connected directly to the
Internet, and a private gigabit NIC on a private non-blocking gigabit LAN.
The private LANs are tunneled over the Internet between the two sites.

Both machines have fairly fast Internet connections. I am able to sustain
around 15 Megabytes per second when downloading data from the Internet (for
example, from a third server I have in Austin) via ftp or http.

When I transfer files between the two machines in Windows Explorer, using
either a mapped drive letter or a UNC path, I find that my average transfer
speed hovers at around 300 Kilobytes per second. What could be causing
this, and how can I resolve it?

Initially I thought it might be a problem with the tunnel, but I got roughly
this same performance when I [briefly] opened the public interfaces on our
routers and did a UNC connection over the Internet using the public IP
address (so the traffic bypassed the tunnel).

Any advice would be greatly appreciated!

Thanks,

-greg