Make sure ALL of your grounds in the circuit are interlinked somehow. One time on a PIC board I forgot to connect the ground, and no matter what the servo just spun towards one side. Once I connected the ground, it was all fixed.
If you are using the visual $50 robot schematic, the one you view in PDF form, there are two wires hidden under the picture of the microcontroller. Uncheck the box to show the microcontroller, and those two wires will be revealed... make sure those are connected.