X.Org 6.9.0 + via8623 (CLE266) + ShadowFB = trouble!

[Krivenok Dmitry]

Hello!
 
I have a strange trouble with my via8623.
This is my device section of config file.
 
Section "Device"
        Identifier "My Video Card"
        Driver     "via"
        #Option     "ShadowFB" "FALSE"
EndSection
 
Note, that ShadowFB option is enabled.
 
All works fine, but...
The trouble appears when I end KDE session or kill X-server via Ctrl+Alt+Backspace.
I see only strange screen flicker (black screen, gray screen, black screen, gray screen, etc...),
instead of kdm greeter.
 
I know about the simple solution of this problem - set "ShadowFB" to "FALSE".
But the price is very low performance (especially in case of image processing).
 
What should I change in my config file to solve this problem?
 
Thank you beforehand!
 
P.S.
This is the part of my config file:
 
Section "Screen"
        Identifier "Screen0"
        Device "My Video Card"
        Monitor "My Monitor"
        DefaultDepth  16
        Subsection "Display"
                Depth 16
                Modes "1024x768"
        EndSubSection
EndSection
 
Section "Monitor"
        Identifier      "My Monitor"
        VendorName      "Unknown"
        ModelName       "Unknown"
        Option          "DPMS" "TRUE"
EndSection
 
Section "Module"
       Load    "ddc"
       Load    "vbe"
       Load    "dbe"
       Load    "dri"
       Load    "extmod"
       Load    "glx"
       Load    "bitmap"
       Load    "type1"
       Load    "freetype"
       Load    "record"
EndSection