Windows 2003 FTP Server MDEL Closing Connection

Has anybody tried to issue an MDEL command to a Windows 2003 IIS FTP server
from an iSeries FTP client?  Everything else works fine.  When I execute an
MDEL command for a large number of files the command executes correctly (and
returns a whole bunch of DELE messages), but the next command I attempt
tells me the connection is closed.

So it looks like this:

MDEL P*.*

227 Entering Passive Mode (10,0,0,1,5,118).
125 Data connection already open; Transfer starting. 
226 Transfer complete.
250 DELE command successful.
250 DELE command successful.
250 DELE command successful.
(...)
250 DELE command successful.

LS

No response received from FTP server. 
No response from remote host; all connections closed. 
Unable to setup for an active data connection to the server, reason code 2. 

I can delete a few files with no problem.  But if I try to delete, say,
several dozen, I get the situation above.  I've set my timeouts everywhere
(on the FTP server on Windows using IIS and on the iSeries using the DEBUG
command in the FTP client) to 1800 seconds, and they seem to be set
correctly; I can pause for several minutes between commands.  I can also
transfer hundreds of files at one time using MPUT.  It's just MDEL that's
causing problems.

Joe