Unexpected behavior when trying to set position of PR2 arm in Gazebo

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Hi,
I am using Fuerte on Ubuntu 11.10.
In Gazebo, I have been using the SetModelConfiguration method suggested here to move a PR2 arm to a certain position. It was working fine for the past several months, but recently I have gotten some unexpected behavior. When I make a service call to /gazebo/set_model_configuration, the robot seems to do a reset to an initial position before trying to set the requested position. This reset also seems to make the robot jump around a little.
I was wondering if anyone else has used this SetModelConfiguration method and has experienced a similar problem.
Thank you

Update: When I try to move an arm through a path in small steps, the shifting is more prominent, and the robot eventually just falls through the floor.

Also, I should have posted the following code before. It is what I am using to call set_model_configuration for moving the arm.

set_model_config_client = rospy.ServiceProxy("/gazebo/set_model_configuration", SetModelConfiguration)
req = SetModelConfigurationRequest()
req.model_name = "pr2"
req.urdf_param_name = "robot_description"
req.joint_names = rospy.get_param("/%s/joints" % ("r_arm_controller"))
req.joint_positions = angles
rospy.wait_for_service("/gazebo/set_model_configuration")
res = set_model_config_client(req)

angles is the list of joint angles specifying the position of the right arm