Player falls through Curved Mesh Colliders

When my player goes onto some curved part of a mesh/model, it falls. Here is a part of the model rendered in SketchUp;

][2]

At the start/bottom of the loop, the player suddenly falls. The same thing happens to other models.

The mesh is a whole model that has a mesh collider. Straight surfaces seem to work but any surface which is in a different angle doesn't work.

The player is a mesh with a box collider. It is controlled by the script used here ( https://www.youtube.com/watch?v=hccdV0plvO4∈dex=5&list=PLmQnFpk1W81tyuEySbOJ4bG6Z1BrS_0hi ).

I've edited it but I think they're non-obtrusive additions.

 // JavaScript Document
 #pragma strict
 var forwardRate : float = 3;
 var turnRate : float = 2;
 
 public var idleAnimation : AnimationClip;
 public var walkAnimation : AnimationClip;
 public var runAnimation : AnimationClip;
 public var jumpPoseAnimation : AnimationClip;
 
 function Update () {
     Speedy();
 }
 
 function Speedy() {
         // tank's forward speed in action
     var forwardMoveAmount = Input.GetAxis("Vertical") * forwardRate;
     // force of the tank's turn
     var turnForce = Input.GetAxis("Horizontal") * turnRate;
     // rotate tank in action
     transform.Rotate(0,turnForce,0);
     transform.position += transform.forward * forwardMoveAmount * Time.deltaTime;
     
    if (Input.GetAxis("Vertical") > 0.2)
        animation.CrossFade (runAnimation.name);
    else
       animation.CrossFade (idleAnimation.name);
 }

I'd be so grateful for anybody who finds an answer.

[2]: /storage/temp/33074-screeny.jpg