There is an app called xvidtune (type it in the xterm) that can fix this sometimes
You need to put the correct specifications for the monitor in the XF86Config file (i.e hysnc vrefresh dotclock etc) (mine is in /etc). There is a modeline calculator here
http://www.dkfz-heidelberg.de/spec/linux/modeline/
though i do not know how good it is. There are also programs you can download to tell you the modelines. I'm sure someone else who knows what all this stuff means can explain it better
there is also a helpfile distributed with X about this though it is pretty technical, i was able to get mine working with *a lot* of trial and error