Thanks for answering.

So i try in english now.

I try to triple-box on one computer with a screen resolution of 1440x900.
My mainchar window is 1280x920 because i like it widescreen, and there are normal graphic settings, runs fine though.
The toons 2 and 3 are on window size 800x600 and all graphic settings reduced to minimum.
I get no performance problems, always use newest graphics card driver.

But everytime i open a second window, regardless if it's toon 1 & 2 or 2 & 3, i get these massive graphical problems in the wow windows.
Also tried OpenGL-Mode, same problems.