When was the last time you checked your drive for fragmentation?
I have found that, occasionally, having a highly fragmented drive can make my system lag as it takes longer for Windows to find files that are fragmented than when it isn't.
Also see if the lag still comes up if you are running just mIRC. It might be that it's not mIRC that's causing your system to lag, but a combination of a number of programs, each one lagging a little bit, and you're just noticing the lag in mIRC.
