I have been experiencing this occasionally as well out of the blue on Windows 7 Ultimate x64.
I am not using any DLLs like these other guys are using. I'm using mIRC 6.35 directly from mirc.com.
I have confirmed that I have no sort of malware of any kind on the machine. I'm highly involved with doing that sort of forensic work.
The freeze is similar to what occurs whenever you force a script into an infinite loop, except the break button does not fix the issue. My issue disappears after about 20-30 seconds when mIRC decides to start responding again. It's kinda strange.