the odd part is that the cpu usage is not constant at all, and only spikes at times.
(Unrelated to network activity, and there's no disk activity since logging is off)

(Also it's not a gdi heap issue since that is set to 40MB here(it's only of 1MB in xp SP1/3MB in xp SP2), which is very unlikely to be reached by just mirc all alone).

I know this is a bit talking in the wind and not helpful but he's the cpu usage chart of mirc for the past few mins for example(resolution .5s):
http://saber.kawaii-shoujo.net/Various/SS-2009-02-08-8.14.45%20PM.png

Note that the spikes are actually full cpu usage since this is a quadcore, so a single threaded program like mirc can not use more than 25%.