Okay, if it is just a basic install of mIRC, we will need to find a way to narrow down the issue.
First, can you try installing a new, clean copy of mIRC, into a new empty folder in your Documents folder, making sure to select the "Run as portable application" option in the installer. You should also uncheck the other options in the installer.
If you then run this copy of mIRC from your documents folder, without changing any other settings in it, do you still see the issue?
How often does it happen? Does it happen within a few minutes of running mIRC?
Does the issue happen when you do something specific, such as connect to a server, or join a channel?
When the issue does happen, does Task Manager show that your available memory is low?