[fargus111111111]

The other thing to consider is how you are telling it where to stop, you will never be able to make it stop dead center every time. You need a range that is acceptable for it to stop within, then move the robot slowly enough that the camera's refresh rate can keep up and it will catch the range. If you want to be more precise without taking more time you can use multiple "stages" where you have it move faster until it is within a wider range, then it slows down to catch a smaller range.
We are running PID control as well, but we run it separate from everything else so we can use it with any movements we make.
Our auton uses a "2 stage" vision targeting system that has been 100% reliable.