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The configuration file is /etc/X11/xorg.config. On some systems you'll find proprietary configuration tools (like system-config-display on RedHat), others have the xorgconfig included.

They need to be launched from a console, with superuser-rights (root). Once your console launched, type su followed by the [Enter]-key. You'll be prompted for the root password (the one you passed to the system during your install procedure).

Once you're root, type xorgconfig , follow the instructions on the screen and answer the questions as precisely as possible. When you're prompted for the refresh rates of your monitor, take a range, kinda like 31 - 90, same for the horizontal sync, like 50 - 160. But first, refer to the manual of your monitor to check out what ranges it supports.

If you have none of these tools, you can edit the file manually using an editor. Refer to the manuals about how to use them. (The manuals can be found by typing man <application_name> in the console). They are usually installed on every Linux system and are called vim, nano, pica or emacs (these are the most common ones).

If you're not feeling at ease with the command line, you can launch a graphical editor as root with the command kate or kwrite (depending on what you've installed).

Open /etc/X11/xorg.config, and look for the lines in the Section "Monitor"

replace the values with the ones corresponding to your monitor.
Look for the Section "Device" and check if the value in the line

matches with your graphic card. If not, we'll see further.

If you have questions, do not hesitate to ask here