X.org Problems on Sun Blade 100 and Sony GDM-20D10

Tonight, I've upgraded my Blade 100 to lenny. Everything went smoothly except X.org. With the new packages and the old config file, the image was out of sync (one sees a flickering mess). After three hours of playing with monitor frequency settings, I had to start thinking. The problem was, the settings reported by fbset -x worked fine on the console. So it had to be the graphics card (years of stable X drivers seem to make one forget about that option). /var/log/Xorg.0.log contained the following lines:

(--) MACH64(0): Reference clock 157.5/11 (14.318) MHz.
(II) MACH64(0): If modes do not work on Ultra 5/10 or Blade 100/150,
        set option "reference_clock" to "28.636 MHz" or "29.5 MHz"

Setting Option "reference_clock" "29.5 MHz" in Section "Device" of /etc/X11/xorg.conf solved the issue. Googling for reference_clock does retrieve some pages that also mention Blade 100 and even GDM-20D10. I had not been able to find any of them before having learned about reference_clock since all my queries contained gdm 20d10 and never blade 100, and I never looked beyond the first ten results. Using gdm 20d10 "out of sync" or blade 100 out of sync would have retrieved some relevant results (like this or this) on the first page. Skimming through more results (especially if there are less than 100 in total) would have also saved time even with an imperfect query.