mIRC is supposed to use the original server password on reconnect. What version of mIRC are you using? Have you defined your bnc in the server list? Having done so may cause mIRC to try that entry which may not have a password set. Can you open a /debug window to verify the password is not being sent on reconnect?
If you're still having trouble you can use the $servertarget identifier in your connect event to send the pass.
