Using xrandr 1.3.2 (supplied by Mandriva 2010.1). In the man page for xrandr the --newmode section does not mention the parameter "DotClock", which should be before "hdisp".
Fix pushed to git master - thanks for the report: http://cgit.freedesktop.org/xorg/app/xrandr/commit/?id=051f912f1c009a2c318214e6b3e86fa5f576d0ff
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