 I have been seeing that behaviour,
  FreeBSD 8.2-STABLE (last build 22/Dec) amd64 (Generic)
  xorg7.5.1 kde-4.7.4

 It can happen with almost any program, it seems to happen sometimes when Xorg has to refresh the screen; at first it seemed like it was a specific program but any program or switching between virtual screens on the KDE/plasma desktop can cause it. Xorg goes to 100% and none of the kde or other X apps can run; nor can you switch to a non-X terminal or kill Xorg from inside.
 I can ssh in and kill Xorg that way.

On Wed Feb 22 Dwayne MacKinnon wrote:
> What versions of xorg, kde & calibre are you running? I'm a bit behind the 
> curve (waiting for a fix to the icedtea & firefox10 program before upgrading 
> to 9.0-RELEASE & kde-4.7.4). 
> 
> I'm running xorg-7.5.1, kde-4.7.3 and calibre-0.8.26 on FreeBSD 8.2-RELEASE 
> i386 and I do not see that behaviour.
> 
> Cheers,
> DMK
> 
> On February 22, 2012 01:12:29 AM Yuri wrote:
>> I have at least 2 programs: calibre (e-book manager) and qbittorrent,
>> which make xorg consume 100% CPU and make system very sluggish when 
run
>> under kde4 (with window effects on).
>> Problem disappears when the same programs are run under gnome or 
xfce2.
>> 
>> Anybody knows what may be causing the problem?
>> 
>> Yuri
