Run xvidtune from a terminal and click 'show'. A series of numbers will be printed like (example from my machine):
"1280x1024" 167.25 1280 1344 1504 1728 1024 1025 1028 1072
It's a modeline. To have it after a reboot you need to add it to /etc/X11/XF86Config (or the configuration file of X.org, depending to what you have). Place for the addition is in the "Monitor" section. In a line before EndSection type Modeline and paste the line you got from xvidtune. The section after modifications may look this way:
Code:
Section "Monitor"
Identifier "monitor1"
HorizSync 30-96
VertRefresh 50-160
Modeline "1280x1024" 167.25 1280 1344 1504 1728 1024 1025 1028 1072
EndSection