I used the top command to figure out which ftp daemon was running. It's proftpd. I used the Proftpd documentation to set up the /etc/proftpd.conf file.
What was odd was
1) although I have 12 virtual sites, only 2 had any config data in /etc/proftpd.conf
2) there was no timeout information whatsoever in /etc/proftpd.conf, yet something, somewhere is causing a very short-fused timeout for FTP
Nevertheless, when I added TimeoutIdle and TimeoutNoTransfer settings to /etc/proftpd.conf, the problem went away.
Thanks, everyone, for asking questions. Those questions led me to enlightenment.
-- hang 10