iTunes loses its link to movies stored on external drive

I have movies stored on an external drive which is connected to my Time Capsule. The movies are part of my iTunes library, but they randomly and continually de-link so that I have to re-add them to the library constantly. It does not generally happen to all movies at once. How can I keep this from happening?

Here's a screenshot.

I got fed up with it happening all the time and moved my movie library to my Time Capsule rather than the external connected to it. Let's see if it makes any difference.