切换图像时，为什么我的幻灯片显示在菜单上方？

I've got a slideshow on my webpage. It switches from images using the following script:

setInterval(function() {
  $('#slideshow > div:first')
    .fadeOut(0)
    .next()
    .fadeIn(1000)
    .end()
    .appendTo('#slideshow');
}, 7000);

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="slideshow" name="slideshow" class="slideshow">
  <div>
    <img src='{ROOT}img/Camerashop24/slide1.jpg' alt="slide-1" class="slide">
  </div>
  <div>
    <img src='{ROOT}img/Camerashop24/slide2.jpg' alt="slide-2" class="slide">
  </div>
  <div>
    <img src='{ROOT}img/Camerashop24/slide3.jpg' alt="slide-3" class="slide">
  </div>
  <div>
    <img src='{ROOT}img/Camerashop24/slide4.jpg' alt="slide-4" class="slide">
  </div>
</div>

However, when the slides are changing, the slideshow is above the dropdown menu until it completely loaded. It then goes behind the menu again. The menu has a z-index of 100, the slideshow has a z-index of 10. If needed, I can give the css of the menu, but that's quite a long list.

Why is the slideshow above the menu when switching?

</div>

Using this code the slides work fine ...

var myIndex = 0;
carousel();

function carousel() {
    var i;
    var x = document.getElementsByClassName("mySlides");
    for (i = 0; i < x.length; i++) {
       x[i].style.display = "none";  
    }
    myIndex++;
    if (myIndex > x.length) {myIndex = 1}    
    x[myIndex-1].style.display = "block";  
    setTimeout(carousel, 2000); // Change image every 2 seconds
}

<div id="slideshow" name="slideshow" class="slideshow">
     <img class="mySlides" src='{ROOT}img/Camerashop24/slide1.jpg' alt="slide-1" class="slide">
     <img class="mySlides" src='{ROOT}img/Camerashop24/slide2.jpg' alt="slide-2" class="slide">
      <img class="mySlides" src='{ROOT}img/Camerashop24/slide3.jpg' alt="slide-3" class="slide">
        <img class="mySlides" src='{ROOT}img/Camerashop24/slide4.jpg' alt="slide-4" class="slide">
</div>

</div>