Some suggestions:
[*] Check to make sure that you are not running any scripts which are also trying to join the same server(s).
[*] Disconnect, type
/remote off and see if the problem goes away.
[*] Make sure that you have no "perform" events (Alt-O/Connect/Options/Perform) that are trying to open up a new
server automatically.
If all else fails, download a new copy of mIRC from this website, install it in a fresh directory, and see if the problem is still there.
HIH
PastMaster