The 10 year old JR in my sedan does that. I'll show ya on Thursday. It happens when the natural frequency of the steering system matches (or is close) to the feedback rate of the servo. Can also be an overly aggressive feedback algorithm causing "overshoot".

http://www.compumotor.com/catalog/cataloga/a36-a38.pdf

I generally touch the tires to stop it if it happens. You can also use thicker grease in the gears to help damp it out.