Why is my AJAX validation code not working correctly?

I have created a site that depends on PHP, AJAX, and Javascript.
The problem here is that the website is unstable, which means that sometimes the validation works and sometimes it doesn't work at all.
The verification code is written in JavaScript. Does this mean that we need more special conditions?

Code to check:

<script language=Javascript>
function Inint_AJAX() {
try { return new ActiveXObject("Msxml2.XMLHTTP");  } catch(e) {} //IE
try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {} //IE
try { return new XMLHttpRequest();  } catch(e) {} //Native Javascript
alert("XMLHttpRequest not supported");
return null;
};

function dochange(src, val) 
{
 var req = Inint_AJAX();
 req.onreadystatechange = function () 
 {
      if (req.readyState==4) 
      {
           if (req.status==200) 
           {    
                document.getElementById(src).innerHTML=req.responseText;
           }
      }
 };
 req.open("GET", "traditional_locate.php?data="+src+"&val="+val);
  req.send(null);
}

window.onLoad=dochange('cities', -1); 


function submitform()
{
 if(document.form1.onsubmit())
 {
    document.form1.submit();
 }
}

function validate()
{
  var p_o_box=(document.form1.p_o_box.value);
  var owner_name=(document.form1.owner_name.value);
  var cities=(document.form1.cities.value);
  var post_office=(document.form1.post_office.value);   


        if(cities == 0)
        {
            alert("Please Choose city");
            document.form1.cities.focus();
            return false;
        }

        else if(post_office == 0)
            {
            alert("Please Choose post office");
            document.form1.post_office.focus();

            return false;
            }

            else if (p_o_box=="")
                {
                    alert ("Please Write P.O.Box");
                    document.form1.p_o_box.focus();
                        return false;                       
                } 

                else if(owner_name=="")
                        {
                        alert("Please Write Owner Name");
                        return false;
                        }

                    else if(p_o_box!="")
                        {
                                var a=new Array;

                             <?php
                                session_start();                                    
                                $zipinfo=$_SESSION[zip_code];                                   
                                $conn=mysql_connect('localhost','root','');
                                mysql_select_db('post_db',$conn);
                                $query="select p_o_box from p_o_box where zip_code='$zipinfo' ";
                                $result = mysql_query ($query , $conn ) or die ( mysql_error ());
                                $rows=mysql_num_rows($result);
                                $n = array();
                                for($i=0;$i<$rows;$i++)
                                {
                                    $data=mysql_fetch_row($result);
                                    $n[] = $data[0]; 
                                }

                                for($i=0;$i<count($n); $i++)
                                {
                                    echo "a[$i]='".$n[$i]."';\n";
                                }           
                                ?>

                                var ss=0;
                                for(i=0;i<a.length;i++)

                                if(a[i]==p_o_box)
                                    {
                                        var ss=1;
                                    }

                                 if (ss==0)
                                    {
                                        alert('not correct p.o. box');
                                        document.form1.p_o_box.focus();
                                          return false;
                                   }
                                else
                                     {  return true;}

                        }                       


    return true;        
}

</script>