jQuery slideshow behaving oddly in Drupal
This because you have two slider block in your durpal website. The second one is hidden in the footer section. Block id is : block-multiblock-7.

Both of these block are having same DOM structure, so in the following scenario your slider will break.

Breaking Scenario:
Main slider -> 5th Image is active.
Footer Slider -> 3th Image is active

Then your condition fails:  

if (nextImg.length == 0) {
            nextImg = $(".header-slideshow-image:first");
            }


Hope you are clear with my point.

Thanks