When attempting to connect to a particular UNIX server (which we found out today was FileZilla), we periodically receive the following connection log:

Finding Host [REMOVED] ...
Connecting to [REMOVED].[REMOVED].39.36:21
Connected to [REMOVED].[REMOVED].39.36:21 in 0.124995 seconds, Waiting for Server Response
Error reading response from server.
Connection closed by remote host.
Host type (1): Automatic Detect

Successful connection attempts appear as follows:

Finding Host [REMOVED] ...
Connecting to [REMOVED].[REMOVED].39.36:21
Connected to [REMOVED].[REMOVED].39.36:21 in 0.078122 seconds, Waiting for Server Response
220 Welcome to the [REMOVED] FTP site.

We have about 10 other FTP uploads that go off regularly without a hitch, so it's most likely not a connectivity issue.  On the server end, the logs show that we're manually disconnecting after a short period of time, so it's hard for me to say its the server's fault either.  My guess at this point is an incompatibility with FileZilla server and WS_FTP Pro 2006 client.  I've tried toggling PASV mode, among other things, but the issue occurs with the same regularity.

Any suggestions for what I should try next?  I'm all out of ideas.