okies, we'll keep to this forum for the meantime, I'll send more easily reproducible bugs directly to firstname.lastname@example.org
Allrighty then, here is a screen-shot of the behaviour using TWM.
In this case, I did not have a lot of printouts in a terminal, but rather had a xforms (which is a widget set) text-label updating with the frame-rate. (some 1000 fps for this particular application).
Look at the fields which appear black. TWM exhibits slightly different behaviour than KDE's WM- when you switch windows most of the time the problem temporarily goes away.
I'm going to try uniprocessor mode next.