jQuery Grid Plugin - jqGrid

Hello,

I'm new to jqGrid and I'm having a little trouble.

I'm trying to constantly refresh a changing data source.

Here's my code:

      setInterval(function (){
         $.ajax({
            url:'datatest.php',
            cache: false,
            success: function(testdata) {
             $("#list1").jqGrid({
               datastr: testdata,
               datatype: "xmlstring",
               colNames:['ColA','ColB', 'ColC', 'ColD'],
               colModel:[
                  {name:'ColA', xmlmap:"COLA"},
                  {name:'ColB', xmlmap:"COLB"},
                  {name:'ColC', xmlmap:"COLC"},
                  {name:'ColD', xmlmap:"COLD"}
               ],
               loadtext: false,
               height:'100%',
               xmlReader: {
                  root:"ROOT",
                  row:"ROWS",
                  page: 1,
                  total: 1,
                  records: "SESSION>rows",
                  repeatitems: false },
               viewrecords: true,
               sortorder: "asc",
               caption:"Table Refresh Test"
               });
         }
      })
      }, 5000);

The table loads fine but will not refresh. I can see the jQuery ajax requests completing in the Firebug Console but the table contents remain stale. Moreover, I added .trigger("reloadGrid") without any success other than I saw the red "Loading..." box every 5 seconds but the data did not update.

Thanks in advance,

Chris