mIRC doesn't have the ability to do what you are saying in-built, so this means you're using some script to do it for you.

It would appear that nobody recognizes what that script is from the description you've thus far given, so...

What script are you using, and have you talked to it's creator about this issue?

If you can't contact them, then follow these guidelines to try narrow down the section of code, then post the relevant portions here that you are having issues with.