That's probably a driver problem. Make sure you have current drivers installed. Nvidias latest proprietary driver for this system is version 325.1, release date: 2013.8.5.
If this doesn't work, try with nouveau, the open source driver, too. During testing, be sure to run MegaGlest from a terminal window so you get to see its output and any warnings and error messages. The system logs you can check for driver errors are ~/.xsession-errors.log (may or may not exist), /var/log/Xorg.*.log and /var/log/syslog.