Enemy floating problem

I am making a horror game, and my zombie, which follows my player, after it walk a few meters it’s Y position increases from 0 to about 2 and it keeps walking. How to fix that?
var target : Transform; //the enemy’s target
var moveSpeed = 3; //move speed
var rotationSpeed = 3; //speed of turning

var myTransform : Transform; //current transform data of this enemy
var isNotDead : boolean = true;
var health : float = 100;
function Awake()
{
    myTransform = transform; //cache transform data for easy access/preformance
}

function Start()
{
     target = GameObject.FindWithTag("Player").transform; //target the player

}

function Update () {
	
	if(health < 1){
	
		isNotDead = false;
		animation.Play("die");
		Destroy(gameObject, 1);
	}
	
	if(isNotDead){
	
	    //rotate to look at the player
	    myTransform.rotation = Quaternion.Slerp(myTransform.rotation,
	    Quaternion.LookRotation(target.position - myTransform.position), rotationSpeed*Time.deltaTime);
	
	
	    
	    var distance = Vector3.Distance(target.position, myTransform.position);
	    if (distance < 3.0f) {
	        animation.Play("attack1");
	    }
	    else{   
	    	//move towards the player
	   		myTransform.position += myTransform.forward * moveSpeed * Time.deltaTime;
	   		animation.Play("walk1");
	    }

	}
}

function ApplyDamage(dmg : float){

	health -= dmg;

}

You move your enemy by translating using Transform.forward (line 38). On lines 27 and 28 you calculate the rotation. I expect the height/Y position of the target is higher than the height/Y position of the enemy. This means the enemy will look up a bit at the target when the enemy is standing on the ground, which means when you translate, the enemy will rise. To fix it you can bring the ‘Y’ position of the vector used in LookRotation down to the level of the enemy:

var v3 = target.position - myTransform.position;
v3.y = transform.y;
myTransform.rotation = Quaternion.Slerp(myTransform.rotation,
Quaternion.LookRotation(v3), rotationSpeed*Time.deltaTime);