Robot randomly stops for a second between moving in a straight line and moving along a curve in gazebo

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I am using ROS jade and Gazebo. I have two pioneer 3dx robots in simulation and they start out moving in a straight line. The robots should continue constantly moving, but switch from moving in a straight line to moving along a curve. The robot on the left does not pause, but makes the smooth transition. The robot on the right pauses for a second in between moving straight and moving along a curve. The code is the same for both robots, except for the fact that each code is talking to a different robot. Can someone tell me why this is happening? Thank you so much!