I have a situation where 20 students come into a school computer lab and log into my FTP server all at once. The first 13 students are able to log in fine. After #13, however, each subsequent log-in attempt is black-holed for 3-4 minutes. After that delay period, another 13 log-ins are possible.

Observations:
1) During that 3-4 minute delay period during which students log-ins fail, if I FTP in from a different IP address, I can log in fine.
2) If I FTP in 13 times from a command line on my PC, I too am black-holed on the 14th attempt.
3) If I FTP in from a command line on my server, I can log in any number of times without fail.

There seems to be something that is seeing multiple FTP log-ins from the same IP and timing out that IP address after 13 successful log-ins. Any idea what it might be?