Problem with ati radeonhd 6250

[nervoso]

Hello,,
I bought a Lenovo G475 notebook
that comes with a radeon 6250

Using the ati driver (the opensource) and
xf86-video-ati-6.14.2 and xorg-7.5.1 it works,
opens the screen at 1366x768  beautifull
on the gnome2.32 desktop
the problem happens when I log out,
the screen returns but with a 1/2 horizontal shift
(in the center of the screen there is the border of
the login image)...
if I reset the X server several times, the screen changes
to black, flickering...   until the login image happens again
and everything is right... (until the next X reset)...
Seems that the X server (or the driver does not resets the
hardwware properly, or maps the screen memory in the
wrong place. The first time (boot) the screen is at the 
correct place, when the X server is reinited (for new gdm login)
the screen memory "shifts"  

How can I get information about how to reset the chip???

Thanks for any help
Sergio 
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