How to monitor this Internet public IP of internal FTP server

Hi all,

We have an internal FTP server that's NATed via Cisco's PIX firewall, so it has a Internet public IP and offers public access.

But some people complain that FTP server is not stable. Depending on internal access (to its internal IP of FTP server), it seems ok. But we can't access its Internet public IP from internal network.

There maybe several possible causes, e.g. carrier's line quality, DNS resolution, etc. We want to monitor its Internet public IP of FTP server to check what the root cause is, how?

Thanks, Case