Victor,
I made some notes on a similar issue some time ago when I had an issue with a monitor that would come up correctly as 1280x1024 if I used a DVI cable, but would come up with 1024x768 if I used a VGA cable. Here's what I have -- this was for Dedora a few releases ago:
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On SouthPole, if the screen resolution doesn't come up as 1280x1024, then as root:
use xrandr to get the display name:
# xrandr
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 2048 x 2048
VGA1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
use cvt to get new mode settings:
# cvt 1280 1024
# 1280x1024 59.89 Hz (CVT 1.31M4) hsync: 63.67 kHz; pclk: 109.00 MHz
Modeline "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
Then enter the following both interactively (to test it) and add it to /etc/gdm/Init/Default for reboots.
xrandr --newmode "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
xrandr --addmode VGA1 1280x1024_60.00
xrandr --output VGA1 --mode 1280x1024_60.00
These get added right after the PATH and OLD_IFS lines in the Default file.
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I found with a later version of Fedora that the file to which to add this is no longer at that location, but I don't know what the correct one is, since I no longer have the issue.
More recently I am also using a TV as a monitor -- I had to jump through some hoops to disable overscanning on the HDMI port used by my computer. Oversanning apparently improves a broadcast picture, but for a computer you lose the edges of the display if it's enabled.
I hope this helps you out.
Richard Wilson