FTP Passive mode Timeout, Windows 2003 Firewall

We have one Windows 2003 server set up for FTP access. When the server's
firewall is on and an FTP client connects to it using passive mode everything
works fine if the FTP directory being accessed is small but if the directory
is large (100's of files) the FTP client will timeout.

If the FTP client connects in active mode all client FTP connections work
fine and if the firewall is turned off on the server all client FTP
connections work fine.

Thank you for your help.
--
Ken