Character controller running around in circles

Hey, first time posting and I hope this is the right forum - I imported the default character controller 3rd person into my scene, but the moment I run the game, my character either starts running in circles or runs in a straight line (usually off the edge of the world).

Hitting other keys will sort of let me steer it, but the moment I let go, it just reverts to going in circles/off the map. Does anyone know what might be causing this?

Anyone have any ideas? This is driving me nuts.