Why does the default Third Person Controller cause my character to spin in place?

I've made sure that I have all three of the necessary scripts attached (Character controller, third person controller and third person camera) and that the camera is tagged as the main camera, but when I press the forward key my character rotates clockwise, and counterclockwise when I press back. Left doesn't do anything at all.

In addition, the mouse-controlled camera is jittery when I try to turn it and snaps back forward. It may be unrelated, but it might not be. I've tested the character without the mouse-controlled camera, so I'm fairly certain that's not the cause of the problem.

What I think might be related is that I don't have a character model with any animations attached. My 'character' is just a couple of cubes stacked together to provide something to look at, as at the moment I'm only trying to make a prototype. Could the fact that it's a basic cube with it's own collision/no animation be related?