Hi,
running KDE 4.4.2 here on a triple head setup (xinerama, no xrandr, no composite).
The monitors are hooked to two nvidia 9400GT cards with last 195.36 nvidia driver.

Being unable to use composite due to the particular configuration, I don't use desktop effects and, at least for some time, the system responsiveness is after all decent.

Anyway it appears that, at random timings (generally between a few seconds and an hour) and without any apparent reason (some RAM is always free and swap is empty), window operations and general redrawing start becoming laggish and delayed _until_ I switch to a textual console (ctrl-alt-F1) and back to X, restoring normal behavior for another random time.
During these events, even moving the focus to another already opened window can take up to two or three seconds. During these window operations the Xorg process points to a high 80-90% CPU time, while it floats around 20-30% in normal conditions.

I found that this misbehavior is much more frequent after starting to use Amarok.

Needless to say that I never faced these problems with xfce 4.6 or other DEs with the same Xorg configuration (didn't try earlier KDE versions).

What can I do?

ciao
--
KDE 4.4.2 on Karmic x86_64.

Last few days without using Amarok:

No more problems. Desktop responsiveness is acceptable again.
Wondering how a single application can lead to so bad desktop performance.

Which applets are you using in Amarok's context view?

They could have bugs which cause excessive repainting or memory use.