Perfectly symmetrical robot has two arms that behave differently

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Hi guys, so I have this robot, its perfectly symmetrical. But yet, when I try to move the joints, the right and left arms behave differently. Most crucially, the left arm after exceeding a certain amount of radians, begins to oscillate and spasm out of control. Everything from the PID values to the inertial values are exactly the same, only that the origin y values are reversed. Eg 5 for left -5 for right. Does anyone know any clue why? Putting an effort limit in the robot's xacro file stops the oscillation but the left and right arms do not reach the same position at the same time... and their end points are different also...I am using velocity_controllers/JointPositionController. Does anyone have a clue why this is occurring ? Thanks