These terminating FTP connections are due of a faulty KeepAlive option of the FTP client.

So the ControlConnection terminates on larger transfers and another transfer is not possible without re-establish the connection.

Under Linux only 2 clients support the KeepAlive mode:

lftp: http://lftp.yar.ru/lftp-man.html (option: ftp:nop-interval)
ncftp: http://www.ncftp.com/ncftp/doc/ncftp.html#sect5 (option: control-timeout)

If your plan supports Webdav, it is an alternative to mount local the Webdav access and copy the data directly.