Random rotation of car

Hello, I'm having an issue that's driving me crazy. I have a scene set up with a plane and a car(made with this script: http://carpe.com.au/slawia/2009/08/unity-wheel-collider-part-3/). The car can go forwards and turn with the WASD keys. My problem is that when I press w and try to go, I get random pushes that force me to rotate a little. This causes my car to not drive in a straight like like expected, but in little arc formations. The wheel colliders have no physics material on them, and the plane is the default Wood physics material. The same thing happens whether there is friction or not. I'm assuming it's a collision problem, but I have no idea!