X.org display problem (ibook g4)

I just installed modular X.org from binary packages (see
http://permalink.gmane.org/gmane.os.netbsd.devel.
packages/28530 ) onto
my ibook G4, in the hope of getting a real dual-head setup -
I
couldn't find any way to make XFree86 do anything but
cloning the
internal monitor to the external one.

The new system mostly works fine, but the internal screen
now has thin
horizontal lines constantly scrolling down.  Does anyone
have any idea
about what the problem might be, and how to fix it?

Magnus