Comment 1 for bug 48289

Try to launch xvidtune from a shell prompt. Click on Left button a couple of time and then click on Test to show what happened. Repeat this procedure if needed and, when ready, click on Show button and exit xvidtune.
Now your console reports a line with some numbers. Open /etc/init.d/xorg.conf (using root privileges) and write under section "Monitor" the word ModeLine followed by the line obtained from xvidtune. Save the file and restart X.org.