Fluctuations in videos working or not

Hi. I have the following issue. On my website I have a page with a video gallery. Now sometimes all videos in the gallery seem to work, but more often only a few work, and the rest show as not functioning. Sometimes all videos I try seem to work in Safari, but not in Firefox browser. Sometimes just the other way around. Does anyone have any advice?

The page I need help with: [log in to see the link]