Strange picture errors


I have openSuSE-11.1 with an ATI video card.
After booting the KDE desktop appears perfectly.
Now I go with Ctrl+Alt+F2 to the console tty2
and return with Alt+F7 to the KDE desktop.
Following strange picture appears:

What happens here? After rebooting the KDE desktop
appears perfectly again. But I may not affect ttyX.