Jittery Collision Detection

I am making a third-person action game and everything went smoothly until I added a cube in my game to test collision/jumping and the collision started jittering/glitching when I continuously walk into it or jump into it. The player is currently a capsule and I tried adding a box collider to it with no luck. Here is the controller script:

 #pragma strict
 
 var moveSpeed : int;
 var rotateSpeed : int;
 var jumpSpeed : int;
 var floor : Transform;
 
 private var isFalling : boolean = false;
 
 function Update() 
 {
     if (Input.GetKey(KeyCode.W))
     {
         transform.position += transform.forward * moveSpeed * Time.deltaTime;
     }
     else if (Input.GetKey(KeyCode.S))
     {
         transform.position -= transform.forward * moveSpeed * Time.deltaTime;
     }
     if (Input.GetKey(KeyCode.D))
     {
         transform.eulerAngles.y += rotateSpeed * Time.deltaTime;
     }
     else if (Input.GetKey(KeyCode.A))
     {
         transform.eulerAngles.y -= rotateSpeed * Time.deltaTime;
     }
 
     if (Input.GetKeyDown(KeyCode.Space) && !isFalling)
     {
         rigidbody.velocity.y = jumpSpeed;
         isFalling = true;
     }
 }
 
 function OnCollisionEnter(collision : Collision)
 {
     var object = collision.transform.gameObject;
 
     if (object.name == floor.name)
     {
         Debug.Log("Colliding with ground");
         isFalling = false;
     }
 }