This is from the wu-ftpd
man page .
Code:
The ftp server will timeout an inactive session after 15
minutes. If the -t option is specified, the inactivity
timeout period will be set to timeout seconds. A client
may also request a different timeout period; the maximum
period allowed may be set to timeout seconds with the -T
option. The default limit is 2 hours.
You may be using a different ftpd, but overall, it might be similar. You just might be hitting the default max timout limit.
Try running a loop to send ten or twenty files at a time to see if it still times out
