PHP - Los checkbox se deseleccionan, usando comet.

<?php

do {

	    echo '<input type="checkbox"  name="contenido[]" value="'.$row_Recordset1['nombre_sala'].'"> '.$row_Recordset1['nombre_sala'].'<br>';

    } while ($row_Recordset1 =mysql_fetch_assoc($Recordset1));

?>

<form  action="checked_on_of.php" name="form1" method="POST">

	      <table width="98%" height="262" border="1" class="cuadrado">

	        <tr>

	          <td align="center" valign="middle"><p>Salas de chat:

	            </p>

              </td>

            </tr>

	        <tr>

	          <td align="left" valign="top"><div style=" OVERFLOW: auto;background:#FFFFFF; WIDTH: 440px; TOP: 48px; HEIGHT: 132px"  id="content" onClick="document.forms[0].submit()"/>Cargando...</td>

            </tr>

	        <tr>

	          <td align="center" valign="top"><input type="submit" name="enviar" id="enviar" value="Enviar"></td>

            </tr>

          </table>

	    </form>

var xmlhttp = function()

        {

            var a;try{a = new XMLHttpRequest();}

            catch(e){try{a = new ActiveXObject('Msxml2.XMLHTTP');}

            catch(e){try{a = new ActiveXObject('Microsoft.XMLHTTP');}

            catch(e){alert('Your browser doesn\'t support ajax');a=false;}

            }}return a;

        }; 

        window.onload = function()

        {

            var a = new comet();

        }; 

        var comet = function()

        {

            var a = new xmlhttp();

            a.open('post',window.location+"?"+Math.random()+"="+Math.random(), true);

            a.setRequestHeader('Content-Type','application/x-www-form-urlencoded');

            a.onreadystatechange = function()

            {

                if(a.readyState == 4)

                {

                    document.getElementById('content').innerHTML = a.responseText;

                    window.setTimeout(function(){

                        a = new comet();                   

                    },1000);

                }

            };

            a.send('algo=algo');

        };