I've accessed Air Forums from a couple different computers in the past where I observed what you report -- the preview blips on and off in an instant. It makes sense that this is an issue with your browser.
Are you on a PC? Do you use Internet Explorer or Firefox? I'd suspect this may happen only on one of the browsers. And I suspect the preview is a Flash plug-in feature. What version of Adobe Flash do you have? Go to
Adobe - Flash Player -- there's a small box that tells you in the middle of the screen. For instance, I have version 10,0,22,87 installed.