Seems like something is trying to attach to that port. Try first of all disconnecting the internet connection (unplug the router fromt he phone line) and see if it stops. If it does, then use either the router's logging or a program like Wireshark to see where the connections are coming from.
You'll need to know your local address and Wireshark will display the source IP address. The use http://whois.domaintools.com to find out who is originating the external connections.