Are you using P2P by any chance? I remember problems with certain P2P program and mIRC running at the same time years back and it may still be an issue. In any case, it's something that is running on your computer because my computer is connected 24/7 and I never have problems with mIRC running.
If you want, you can display all of the processes listed in the Processes tab of your Task Manager (Ctrl-Alt-Del to bring it up).
