How to solve that gazebo freeze ubuntu?

edit

I run several times to add object to gazebo, and sometimes my ubuntu just freezed so that my mouse and keyboard all couldn't work anymore.

My system is ubuntu 64 bits with ROS fuerte. I use optirun to run roslaunch.

I use Geforce 630M GT 2GB.

Here is my launch:

  <launch>

    <!-- start gazebo with an empty plane -->
    <param name="/use_sim_time" value="true" />

    <node name="gazebo" pkg="gazebo" type="gazebo" args="/home/sam/code/ros/temp/t1.world" respawn="false" >
    </node>

    <!-- start gui -->
    <node name="gazebo_gui" pkg="gazebo" type="gui" respawn="false" output="screen"/>

  </launch>

Here is my t1.world:

  <launch>

    <!-- start gazebo with an empty plane -->
    <param name="/use_sim_time" value="true" />

    <node name="gazebo" pkg="gazebo" type="gazebo" args="/home/sam/code/ros/temp/t1.world" respawn="false" >
    </node>

    <!-- start gui -->
    <node name="gazebo_gui" pkg="gazebo" type="gui" respawn="false" output="screen"/>

  </launch>

  sam@sam:~/code/ros/temp$ cat t1.world 
  <?xml version="1.0"?> 
  <gazebo version="1.0">
    <world name="default">
      <scene>
        <ambient rgba="0.5 0.5 0.5 1"/>
        <background rgba="0.5 0.5 0.5 1"/>
        <shadows enabled="false"/>
      </scene>

      <physics type="ode">
        <gravity xyz="0 0 -9.8"/>
        <ode>
          <solver type="quick" dt="0.001" iters="10" sor="1.3"/>
          <constraints cfm="0.0" erp="0.2" contact_max_correcting_vel="100.0" contact_surface_layer="0.001"/>
        </ode>
      </physics>

      <!-- Ground Plane -->
      <model name="plane1_model" static="true">
        <link name="body">
          <collision name="geom_1">
            <geometry>
              <plane normal="0 0 1"/>
            </geometry>
            <surface>
              <friction>
                <ode mu="10.0" mu2="10.0" fdir1="0 0 0" slip1="0" slip2="0"/>
              </friction>
              <bounce restitution_coefficient="0" threshold="1000000.0"/>
              <contact>
                <ode soft_cfm="0" soft_erp="0.2" kp="1e10" kd="1" max_vel="100.0" min_depth="0.0001"/>
              </contact>
            </surface>
          </collision>

          <visual name="visual_1" cast_shadows="false">
            <geometry>
              <plane normal="0 0 1"/>
            </geometry>
            <material script="Gazebo/Grey"/>
          </visual>
        </link>
      </model>
  <!-- Box -->
    <model name='box_model' static='0'>
      <link name='body' gravity='1' self_collide='0' kinematic='0'>
        <origin pose='0.000000 0.000000 0.500000 0.000000 -0.000000 0.000000'/>
        <inertial mass='1.000000' density='1.000000'>
          <inertia ixx='1.000000' ixy='0.000000' ixz='0.000000' iyy='1.000000' iyz='0.000000' izz='1.000000'/>
        </inertial>
        <collision name='geom' laser_retro='0.000000'>
          <geometry>
            <box size='1.000000 1.000000 1.000000'/>
          </geometry>
          <origin pose='0.000000 0.000000 0.000000 0.000000 -0.000000 0.000000'/>
          <surface>
            <bounce restitution_coefficient='0.000000' threshold='100000.000000'/>
            <friction>
              <ode mu='-1.000000' mu2='-1.000000' fdir1='0.000000 0.000000 0.000000' slip1='0.000000' slip2='0.000000'/>
            </friction>
            <contact>
              <ode soft_cfm='0.000000' soft_erp='0.200000' kp='1000000000000.000000' kd='1.000000' max_vel='100.000000' min_depth='0.001000'/>
            </contact>
          </surface>
        </collision>
        <visual name='visual1' cast_shadows='1' laser_retro='0.000000' transparency='0.000000'>
          <geometry>
            <box size='1.000000 1.000000 1.000000'/>
          </geometry>
          <material script='Gazebo/WoodPallet'/>
        </visual>
      </link>
      <origin pose='0.000000 0.000000 60.000000 0.000000 -0.000000 0.000000'/>
    </model>

      <light type="directional" name="my_light" cast_shadows="false">
        <origin pose="0 0 30 0 0 ...