Game object position is stuck when using mathf.sin,Object's position stuck when using Math.sin

Hello!

I am trying to making one of my objects bob up and down while at the same time moving left across the screen. The code I currently have achieves this, but now it keep snapping the object back to 0 on the y axis and begin bobbing and moving from there. If I manually move the object, or try to set the code to a different starting position, it will still throw it straight back to 0 on the y. Here is my code:

public float horizontalSpeed; public float verticalSpeed; public float amplitude;

 private Vector3 tempPosition;

 void Start () 
 {
     tempPosition = new Vector3 (4.32f, 2.28f, -0.66f);
     //tempPosition = transform.position;
 }

 void FixedUpdate () 
 {
     tempPosition.x += horizontalSpeed;
     tempPosition.y += Mathf.Sin(Time.realtimeSinceStartup * verticalSpeed)* amplitude;
     transform.position = tempPosition;
 }

I am new to Unity and coding so there is probably a simple solution to this but I cant seem to figure it out. I tried applying different methods from other peoples' questions but I keep running into the same problem.

Thanks in advance for your help!