Used to get this a lot in the old days - don't know if it's still available, but try running sudo xvidtune in a terminal.
If it works, click Auto, then adjust using the buttons.
If it gets where you want it, click Show and paste the modeline entry into your Xorg.conf
However - you may want to make sure your monitor refresh rates are set correctly in xorg first as this can have a similar effect - esp. refresh rates!