after changing the port setting
avail on for first dcc send
then mIRC use port increase by 1 every time
e.g.:
dcc send use port 1234
set 1024 1024
now:
dcc send use port 1024
and then:
dcc send use port 1235

This is normal. What happens is that even if a socket on a certain port is freed, Windows may mark it as unusable for a period of time.

If mIRC is unable to open a socket on a certain port for a dcc transfer, it will automatically default to trying any available port.