You may wish to try backing up whatever you have as your /etc/X11/xorg.conf file with sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak and then editing the file to include your new resolution. Use the format as shown below, with the figures you need, and then restart. If you are lucky, you may get the res you are looking for.
If you current xorg.conf has other sections that are already in it, don't delete those; the bit you may need is the Section "Screen" part
# xorg.conf (X.Org X Window System server configuration file)
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg
Identifier "Configured Video Device"
Identifier "Configured Monitor"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"