Back Button and Embedded Data

Hi there!
Any ideas on how to 'erase' and 'rewrite' embedded variable values in javascript.
I have a lot of display logic (to avoid using branched logic), but the display logic doesn't seem to work if I go back and change a response. And sometimes entire sections skip for no apparent reason. :-(
I have tried using this...
Qualtrics.SurveyEngine.addOnPageSubmit(function(type)

{

 var that = this;

 if(type == "next")

  {

      Qualtrics.SurveyEngine.setEmbeddedData("Var1", jQuery("#" +that.questionId …
and also..
Qualtrics.SurveyEngine.addOnUnload(function()

{

 var that = this;
  {

      Qualtrics.SurveyEngine.setEmbeddedData("Var1", jQuery("#" +that.questionId …

Any idea would be well appreciated!
J