not really a solution to the problem - I suspect your problem is browser and bandwidth. You could always navigator buttons in individual discussions with http://vanillaforums.org/addon/pagenavigator-plugin. and you could modify the plugin to add arrows to the discussion topic page.
Do you have vanilla embedded? I believe this is often a timing issue the discussions topics are not fully retrieved by the time the browser writes the code. Also, I've seen this happen more often than not on IE browsers. got more details? of your environment.