Can't switch tty after logged in kde for a while

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https://bbs.archlinux.org – I get only a black screen with Ctrl+Alt+F1(or 2~6). This happens a while after logged in kde.I can login and exe commands in those black tty's, there are reactions to "halt" though I couldn't see anything from the screen.Actually "halt" and "reboot" do not work correctly (they closed everything but don't turn off the power) after tty switching gets broken.In the first a few minutes logged in kde, everything works fine.I have tried to reinstall video card drivers, and change different ways of starting kde (in inittab or rc.conf or by hand). The probl (HowTos)