None of the videos at that site worked for me. Looking through the page code I couldn't find a URL for the QuickTime files.
Only the page author can fix it.
QTKirk, source code appears to be pre-cached by previous web page before jumping to this page. In any case, the streaming server appears to be down or at least off-line when you try to access the video stream directly which probably explains both the displayed question mark and the "now you see it—now you don't" nature of access mentioned by the OP here.
It still means the authors need to fix things and it's not a problem caused by QuickTime.
Or it may only be routine maintenance being performed over the weekend. In any case I would try again on a weekday to see if content is then available. Also noted that local web server stored content is still available.
Take care and have a good weekend. I've got to go start on the yard... Argh...!