Since nobody asked in neither this thread nor the other thread, I'll ask, which mIRC version are you using? Also, do you use nick colors? I ask because there was an issue which was fixed in 6.3.

From versions.txt
16/08/2007 - mIRC v6.3

Changes:
..
79.Fixed nick color list freeze issue, instead of updating all nicks
immediately when colors change, now only updates them when they
are visible.