This should be expected to happen from time to time, when there are a very limited number of ports to choose from, and especially after having performed multiple sends. sockets take time to "cool down" before they can be used again, and if mIRC feels that none of these sockets are available it simply tries the next best thing.
You should also be aware that other programs might be making use of the ports you have selected for mIRC. Ports 1024 through 5000 are shared application ports, and should never be dedicated to the operations of a single application. I strongly urge all users who need to define a port range through their router or firewall, to select a range above 12000 and below 65000. This will both limit interference with other programs, breaking other programs, and will make it harder for someone to predict and intercept.