Try downloading and installing this driver:
http://www.acerpanam.com/synapse/for...ords=&areaid=7
Uninstall your current driver in device manager. Then, boot into safe mode and install the SiS driver. Reboot and start Windows normally and see if you can get that resolution.
What option in power strip is disabled (take a screenshot). You should be able to enter a custom resolution and refresh rate.