Setting Specific Scan Rate in xorg.conf: Puzzled

[Kent Borg]

I have a new LCD panel and I can't get a stable display on it--text
and vertical lines flutter.  Various playing with "dpkg-reconfigure
xserver-xorg" and pressing the "Auto" button doesn't seem to have
fixed it, but one thing does have me really puzzled: Why can't I set
the horizontal and vertical scan rates by editing /etc/X11/xorg.conf
and changing "HorizSync" and "VertRefresh"?

One of the "factory presets" listed in the manual is 63.89kHz and 60
Hz.  I set HorizSync to "63-64" and VertRefresh to "59-61".  This way
X will have to choose the preset I want right?  Wrong.  When I fire up
X and ask the monitor what it is being fed it says 74.6 Hz and 79 Khz.
Why isn't it listening to me?

I am running a fresh install of Dapper Flight 6 (with updates) and a
Gem GM-171B 17" display (that got good reviews on Newegg).  The
computer is an HP, with 82915G/GV/910GL onboard video (which is
accelerated, can run Planet Penguin Racer!), there are no errors in
/var/log/Xorg.0.log.

Ideas?

Thanks,

-kb