Radeon M6LY very dark @ 16bit (IBM TPX31)

[doozer]

After installing F12 i tried to change xservers colordepth from 24 to 16 bit to get 3D acceleration(i have done this succesfull on F1-F11).
But on F12 i get a very dark picture - only the mouse curser has full brightness (-> no problem with the backlight, maybe mouse cursor is not effected because it is hardware accelerated??)
@24bit screen has normal brightness.
Problem exists on internal TFT and on external connected monitor.
Changing gamma with xgamma don't solve the Problem.


Hardware:
IBM Thinkpad X31
ATI Radeon Mobility M6 LY (=RV100 series)

xorg.conf:

# Xorg configuration created by system-config-display

Section "ServerLayout"
        Identifier     "single head configuration"
        Screen      0  "Screen0" 0 0
        InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

Section "InputDevice"

# keyboard added by system-config-display
        Identifier  "Keyboard0"
        Driver      "kbd"
        Option      "XkbModel" "pc105"
        Option      "XkbLayout" "de"
EndSection

Section "Monitor"
        Identifier   "Monitor0"
        ModelName    "LCD Panel 1024x768"
        HorizSync    31.5 - 48.0
        VertRefresh  56.0 - 65.0
        Option      "dpms"
EndSection

Section "Device"
        Identifier  "Videocard0"
        Driver      "radeon"
EndSection

Section "Screen"
        Identifier "Screen0"
        Device     "Videocard0"
        Monitor    "Monitor0"
        DefaultDepth     16
        SubSection "Display"
                Viewport   0 0
                Depth     24
                Modes    "1024x768"
        EndSubSection
        SubSection "Display"
                Viewport   0 0
                Depth     8
                Modes "1024x768"
        EndSubSection
EndSection

Anybody with the same problem or any idea how to fix it?