enemy movement pattern

[ChaseNYC]

I don't know why but I cannot seem to figure out how to make a cosine(math term) like movement of an enemy... I'm just really stupid i guess... i tried the following code which just leads to some really weird movement:

//basically the code is supposed to make the enemy wave back and forth
//as it moves down the screen, creating a cosine like effect
//this.startX = where on the x axis the enemy starts
//this.dFrmStart = the distance from the startX
//this.maxDist = how far the enemy should go before it turns around
//bgSpeed = background constant speed
//this code tries to check how close enemy is till needing to turn and modifies
//the enemies speed accordingly to make a curved looking turn
//problem is, it doesnt work...
                            if(this._x>=this.startX){
				this.dFrmStart=this._x-this.startX;
			}else if(this.startX>this._x){
				this.dFrmStart=this.startX-this._x;
			}		
			if(this.dFrmStart>this.maxDist){
				this.speedX=this.speedX*(-1);
			}
			
		if(this.dFrmStart==0){
			this.dFrmStart=1;
		}
			this.speedMod = (this.dFrmStart/this.maxDist)
			this.speedMod = 1 - this.speedMod;
			if(this.speedMod<0.2){
				this.speedMod=0.2;
			}
			if(this.speedMod==0){
				this.speedMod=1;
			}
			
			this._y+=(this.speed*this.speedMod)+bgSpeed;
			this._x+=this.speedX*this.speedMod;

I am kinda ashamed to show my attempt but just wanted people to see what I tried and that I'm not just asking for code without doing any work... Any tips would be wonderful...

-ChaseNYC