What video card do you have? If you have an nvidia card, do the following in the terminal:
sudo gedit /etc/apt/sources.list
Then, a text editor should pop up. Scroll down and uncomment(by taking out the # in the front) the line that has "multiverse" at the end. JUST THAT LINE, nothing else. Then save it and close it.
This do this:
sudo apt-get update && sudo apt-get install nvidia-glx
It should install the nvidia drivers. When that's done, reboot and try playing tremulous again. If the same thing happens, do this:
sudo gedit /etc/X11/xorg.conf
A text editor should pop up. DO NOT CHANGE ANYTHING UNLESS I TELL YOU TO DO SO! Look for something like this:
Section "Device"
Identifier "NVIDIA Geforce 7800GT"
Driver "nvidia"
BusID "PCI:5:0:0"
VideoRam 262144
EndSection
Thats my video card section in my xorg.conf. Yours will have the Driver set to "nv". Change it to "nvidia", like mine, and save/close. Reboot and try tremulous again.
IF YOU MESS UP YOUR XORG.CONF, YOU MIGHT NOT BE ABLE TO BOOT INTO A GRAPHICAL USER INTERFACE. TO FIX IT, LOGIN AND TYPE "sudo dpkg-reconfigure xserver-xorg", OR EDIT IT BY HAND TO RESTORE THE GUI.