IIS 6 FTP service randomly disconnects clients

[chpicker]

We have a web server running IIS 6 on Server 2003 SP1. We use the FTP portion of IIS to upload the content to the web site. The problem we run into is that, when we try to upload a lot of files, the connection to the server will die at a random spot. If I'm using Internet Explorer to do the upload, I get the following message box:

An error occurred copying a file to the FTP Server.  Make sure you have permission to put files on the server.

Details:
The connection with the server was reset

If I use a real FTP client (SmartFTP) I get this:

An existing connection was forcibly closed by the remote host.

The web server is outside our ISA 2000 firewall and has a public IP address.

Any ideas what might cause this? The only workaround we have found so far is to just keep trying or try just doing smaller batches of file transfers. Eventually it all gets uploaded.

Ian