kwin in KDE4 causes CPU overload

Hello,

At least once a day (I shutdown at night) kwin, in KDE 4.2.4, causes 50% CPU
(in System Activity) utilisation (dual CPU therefore 1 CPU is 100%). If I
turn off the Desktop Effects (SHIFT+ALT+F12) then kwin returns to the normal
0-4%. When I reactivate Desktop Effects kwin still remains below 15%.

While kwin is spinning out of control other apps become very very slow.

I am experiencing this on two system: one with radeonhd driver and the other
with nvidia's Xorg driver.

Anyone know what's wrong or a fix/workaround? Could this be GTK related? I
use KDE and I tend to notice the slowdown effect more in Firefox and The
Smart Package Manager.

Thanks!