SERV-U Gateway

When I connect to file sharing using the the MFT server internally meaning I don't go through the Gateway, it works fine every time.  When I connect to it using the Gatway it works about 25% of the time.  The rest of the times it gives you the login prompt and when you login the MFT server will show that you connected but then it either just hangs there spinning until gives up or it will pop you right back to the login prompt.  Has anyone experienced anything like this.  Keep in mind this is not an issue with firewall settings as those have been checked and re-checked and if it were a firewall issue it wouldn't work some of the time and not others.

Thanks for any suggestions - Joe