Try putting the dynamic text box inside a movieclip. Give this mc an instance name and place your loadVariables script inside this mc on frame1, together with the dynamc text box. Add this just before the loadVariables script-
This will append a randomo value to the url string when you initiate the loadvariables action, effectvley making sure Flash loads a 'fresh' script every time. Now, lets say your PHP script is called 'fetcher.php', your loadvariables action should be-
loadVariablesNum ("fetcher.php", this, "GET");
Now, extend the timeline of your mc over, say, 40 frames. This means that every 40 frames the mc will loop, hit frame1 again and hopefully return a random word.