How do I add a video time trigger to control other symbols?

I would like to use a video to trigger when other Symbols appear on the Stage at different times in the video. I've used the following code:

       var vid = sym.$("VideoSymbolName")[0];

       vid.addEventListener("timeupdate", function() {

             if (this.currentTime >= 4) {

               // Do Something This code loads a symbol into a container

                  var firstS = sym.createChildSymbol("SymbolName", "ContainerName");

             }

        }, false);

However, that loads the picture repeatedly, and several instances appear on the stage, so I tried using this code:

       var vid = sym.$("VideoSymbolName")[0];

       vid.addEventListener("timeupdate", function() {

             if (this.currentTime >= 4) {

               // Do Something This code loads a symbol into a container

                var con= sym.$("PicPortal");

                con.empty();

                  var firstS = sym.createChildSymbol("SymbolName", "ContainerName");

             }

        }, false);

However, that makes the loaded Symbol "flash", as it is constantly loaded and unloaded in the container.

Any other ideas?
Thanks