Sample Assets FP bobbing

I've got a problem with the FP camera from the Sample Assets, if i walk over a plane it's fine but if I walk over a terrain it judders (video below).

The problem is in this code

 // vertical head position "spring simulation" for jumping/landing impacts
         springVelocity -= velocityChange.y;                  // input to spring from change in character Y velocity
         springVelocity -= springPos*springElastic;           // elastic spring force towards zero position
         springVelocity *= springDampen;                      // damping towards zero velocity
         springPos += springVelocity * Time.deltaTime;        // output to head Y position
         springPos = Mathf.Clamp( springPos, -.3f, .3f );     // clamp spring distance
 // snap spring values to zero if almost stopped:
         if (Mathf.Abs(springVelocity) < springVelocityThreshold && Mathf.Abs (springPos) < springPositionThreshold){
             springVelocity = 0;
             springPos = 0;
         }

If I turn down the springDampen it stops but the effect is lost.

Has anyone came across this problem or know why this would happen over terrain?

Cheers.

VIDEO