FTP 'get' very slow on Windows Server 2003, but not on XP

We have a process, running on one of our servers (a Windows Server 2003
SP1) that automatically transfer files from a public FTP server
(ftp.ncbi.nlm.nih.gov). Two months ago it was working fine.

But now the file transfer starts fast, but quickly becomes *very slow*
as the transfer proceeds (essentially it stops, down to an average of a
few hundred bytes per second by the time it has transferred about 200
Kbytes). Eventually the transfer simply stops .. although the
connection to the remote sever seems to still be open.

I have reproduced this by manually using the Windows FTP command line.
I monitor the transfer using the Task Manager, looking at the column
"I/O Write Bytes". Bytes are flowing well initially, but then slows
down to almost nothing. Using PerfMon to look at Bytes received/sec,
there are initially "bursts" of data every second or so, but then the
bursts get smaller and farther and farther apart.

This happens on two different Win2003 servers in our company (both are
Domain Controllers).
But the strange thing is that on two different desktop machines
(Windows XP SP 2) it works fine!! Transfer of a 7Mbyte file proceeds
with no problem. Same network, same ftp remote server, same file!

I was thinking that maybe file scanning by real-time virus protection
could be the problem (using Symantec) ... but I tried disabling that on
the server, no change (plus it was always running on my desktop WinXP
machine, causing no problems).

Any ideas???