If the page doesn't load completely, thickbox will not work.
If there is a net hiccup for any reason and you instantly click on the detailed images, it will take you to the image page, not thickbox.
Unless you are specifically going onto a pgae to test detailed images, NO CUSTOMER will ever do this. Thanks for wasting my bandwidth to test your problem.
The solution (and I've done this) is to optimize your page load speed and make sure your enitre page loads fast. Lose the elements that slow you down.