I've experienced the cpu spike as well, and have been able to reproduce it many times.
Easiest to reproduce by switching from another program to mIRC and hovering over an switchbar button.
CPU load is focused on (on my computer) cores 3 and 4, with 2 barely registering and core 1 not registering any load.
Resource Monitor shows that half of the load comes from mIRC, and the other half is from explorer.exe.
Hovering over status button is exempt from this bug, it seems.
2f63a83968f9586fe4fb48134253619c 6.35 Vista 5 0
Windows 7 Professional 64-bit
Core 2 Quad