Problem with FTP server behind BM NAT

We are having an issue with our FTP site, which is located behind a
BorderManager firewall using NAT.

A few users, but certainly not all users, receive this error when trying
to access our FTP server.

Error: Connection timed out
Error: Failed to retrieve directory listing

Most users connect just fine. No errors.

I have tried to connect from home to our FTP server. No problem. But, if
I connect to the FTP server through our Guest DMZ that provides internet
access to guests in our building via a Cisco Pix, I also get the error
messages. Strange thing is... when the error occurs, the FTP server
seems to be answering the FTP commands by providing the PRIVATE IP address
and not the PUBLIC IP address. Thus, the traffic is dropped at the
firewall. The FTP server accepts the password and the login name, but
fails when trying to provide the directory listing of the FTP site to the
client.

I have Static and Dynamic Mode turned on for NAT.

Any ideas what would cause this ?

The FTP server is not the same server as the BorderManager server.