The oscillation of a joint with no command and zero gains [closed]

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Hi,

I have loaded my robot into Gazebo. This robot is 6 DOF. You can see the files from here. I have tuned PID parameters of ros_control for five joints. But, tuning joint 4 is not really easy. The behavior of joint 4 is strange too. When the input is in the following range (1.5<$input$<2 and -2<$input$<-1.5) the joint moves correctly. Otherwise, the error signal oscillates and the process_value signal does not oscillate. The reason to this issue is not clear to me.

Would you please give me a hint how to do that.

To run the code easily please enter the following lines:

1) run gazebo:

roslaunch denso_gazebo denso_world.launch

2) run rqt to show the figures and commands:

roslaunch denso_control denso_rqt.launch

The distro of my ROS is indigo running on Ubuntu 14.04.

Thank you.