i believe there is another reason since i've experienced it
mirc will use a rather large amount of cpu on any message, and this gets big if user lists are big(particularily join/parts)
being on more than 140 chans on 12 networks, i noticed mirc uses an average constant of 6% cpu constantly, just because of the joins/parts and without any script, on a PIV 2.4ghz. But the thing is that joins&parts are very unregular, and sometimes there will be spikes at 12% cpu usage, and other down at 0%, which creates the stutter in games. Putting mirc to a lower cpu priority indeed solved my stutering prob, but created massive timeout probs, since mirc didnt get enough cpu time.
