model contact with ground issue

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Hello,

I am running Gazebo7 ver 7.16 in Ubuntu 16.04 and Ros Kinetic.

I have a four wheeled robot that I am trying to simulate in Gazebo:

The two front castor wheels are behaving "strange". I believe there are two issues and they are related:

Issue #1 - Dancing / moving / rotating I took pictures of the model in with no commands being given to it, and if you look at the two images below you will see the two front castor wheels are in slightly different positions (they have moved). The two wheels keep doing this motion where they turn inwards, and then outwards. this motion is done by both wheels and it takes about a second to do (inward to outward about a sec, and outward to inward about a sec). This motion never stops. I am hoping to eliminate this motion:

Here is a close up of the right wheel showing this motion (right wheel relative to robot):

Issue #2 - Contact with ground - I selected "contacts" from the "view" menu and the part of the wheel that makes contact with the ground changes many, many times a second. To show you what I mean I paused the simulation and took images as I pressed the "step" button on the bottom toolbar:

Any suggestion on how my urdf has to change? what tags do I need to add to make the robot more stable?

My urdf is just shy of 800 lines, so I only included the items relevent to the right wheel (as the left wheel is identical).

reference image:

URDF:

  <link name="right_castor_mount_link">
    <collision>
      <geometry>
        <box size="0.1145 0.1 0.0057"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </collision>
    <visual>
      <geometry>
        <box size="0.1145 0.1 0.0057"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black"/>
    </visual>
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="0.25"/>
      <inertia ixx="0.000209010208333" ixy="0" ixz="0" iyy="0.000273807083333" iyz="0" izz="0.000273807083333"/>
    </inertial>
  </link>

  <joint name="base_to_right_castor_mount_joint" type="fixed">
    <parent link="base_link"/>
    <child link="right_castor_mount_link"/>
    <origin xyz="-0.09445 -0.344 -0.03265"/>
  </joint>


  <link name="right_castor_dummy_link">
    <collision>
      <geometry>
        <box size="0.02 0.02 0.02"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </collision>
    <visual>
      <geometry>
        <box size="0.02 0.02 0.02"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="orange"/>
    </visual>
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="0.25"/>
      <inertia ixx="1.66666666667e-05" ixy="0" ixz="0" iyy="1.66666666667e-05" iyz="0" izz="1.66666666667e-05"/>
    </inertial>
  </link>


  <joint name="right_castor_mount_to_right_castor_dummy_joint" type="continuous">
    <axis rpy="0 0 0" xyz="0 0 1"/>
    <parent link="right_castor_mount_link"/>
    <child link="right_castor_dummy_link"/>
    <origin xyz="0.0 0.0  -0.01"/>
    <dynamics damping="0.0" friction="1.0"/>
  </joint>


  <link name="right_castor_wheel_link">
    <collision>
      <geometry>
        <cylinder length="0.049" radius="0.07925"/>
      </geometry>
      <origin rpy="1.57079632679 0 0" xyz ...