Third Person Controller Script Turning Randomly Up and Down Hill [PLEASE HELP]

I downloaded Unity Technologies' Standard Assets, and deleted mostly everything necessary until I had just the Third Person Character things and some of the other necessities (Utilities, etc.). Then I ran around with the ThirdPersonController prefab on a terrain that has a slope. When I move forward on level ground, I go completely strait... However, when I go up and down any terrain slope and press just the "w" key to go straight forward, the character rotates rotates left or right slowly while moving forward... WHY IS THIS?! The running forward is controlled completely by the running forward animation, btw.

Thanks, Kenne