setInterval reliable?

I am calling setInterval on a loadThumbs function that basically loads up thumbnails jpegs.
It seems when I test it locally in Flash, the thumbnails all load fine, one after the other, but when I test in the browser, sometimes, some of the thumbnails do not show up, so I have gaps in my row of thumbs. I have tried increasing the interval time from 200 to 700 to 2000 and this makes no difference.

What could be the problem?