Snapping Target to Mouse Position

I've looked through some other answers and I've only found something close to what I needed. I'm looking for help with a script that allows a target to snap to the mouse position throughout the entire game.

This is what the target looks like when not in play mode. I have attached the following script (Credit: @tanoshimi ) in an attempt to have the target go to the current mouse position, but when I enter play mode, the target seems to be rapidly alternating between two positions. It is alternating between 5.05 and -2.7 in the z-axis, and alternating at a range of units in the y-axis based on the current position of the camera in the y-axis:

 {
     void Update () 
     {
         Vector3 temp = Input.mousePosition;
         temp.z = 7.75f; 
         this.transform.position = Camera.main.ScreenToWorldPoint(temp);
     }
 }

Any help with this would be greatly appreciated. In case this piece of information helps, the camera is currently following the cube along the x and y axes and the cube is clamped in the z-axis.

Thanks!