Version:           unknown (using KDE 3.5.5, Debian Package 4:3.5.5a-1 (testing/unstable))
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.18-1-686

The problem only appeared after my last update from Debian unstable, and appears only to present itself when the window manager mouse-focus-setting is set to "focus follows mouse".
Very often, with "focus follows mouse", the window that is currently under the mouse will "flutter" uncontrollably, i.e. it appears to gain and lose focus again and again in rapid succession.  During the flutter, the responsiveness of the desktop becomes sluggish.
Rapidly clicking on the title bar seems to halt this behavior, though it reappears when the mouse is moved off and then back on the window.
I changed the window manager mouse-focus-setting to "click to focus" in the mean time, and the problem appears not to occur.