force ground truth value keeps bouncing between positive and negative

edit

I use a gazebo_ros_f3d plugin to check force ground truth value of one of arm link, gripper_roll_link. Here is the part of code of robot.xacro

<!--//////////////add gazebo_ros_f3d contcat sensor!!!!!!/////////////////-->
     <gazebo>
              <plugin name="grasping_frame_contact_sensor_controller" filename="libgazebo_ros_f3d.so">
                    <alwaysOn>true</alwaysOn>
                    <updateRate>30.0</updateRate>
                    <robotNamespace>/seven_dof_arm</robotNamespace>
                        <bodyName>gripper_roll_link</bodyName>
                        <topicName>gripper_roll_link_force_ground_truth</topicName>
              </plugin>
        </gazebo>

Problem: Why the value keeps bouncing between positive and negative ? It is extremely unstable.

My guess is the inappropriate setting of moments of inertia of links where all the values of moments of inertia are made-up. Does it make sense?