check your probe output like this reference
http://gnuman.com/guides/the_chronic...uring_x_3.html
normally in /etc/X11/xorg.conf or xf86free file
if you have no modelines in that file you can after understanding the warnings run
xvidtune in a console or do a search here or google for your monitor for a correct file as above that gives the correct modlines
I don't have your monitor or FC so can not help any further