mIRC still shouldn't use that much CPU. But it's almost guaranteed to be related to a script or DLL that is hanging. If a clean install of mIRC is used (that means no settings saved, no scripts or DLLs installed, not a hacked version, etc) and it still hangs, then we'd need to know what happens before it hangs. Does it hang after X minutes, or is it random? Does it hang immediately? What happens if you hit Ctrl-Break multiple times with mIRC in focus? Does any kind of error message appear?
Also, there are many cases where one program hangs because of a problem in a completely different program that appears to be working normally. I've seen that happen many times in Windows. Usually, when that is the case, closing the one that appears to be hanging will make the high CPU usage switch to a different program. That's a clear indication that it's something else causing programs to hang and not the fault of the hanging programs.
One last thing... when the screen freezes, does that mean you cannot type anything in the edit line? Some networks will stop sending you channel text if you are idle too long, making it appear that the screen froze. Basically, if you were idle long enough, no more channel messages would appear. Once you said something again, then you would be allowed to receive messages again.
