Google Maps API 3 Load Markers from MySQL from current position

I'm trying to make a map with google maps api, that displays a lot of markers from a MySQL Database. I did nearly everything as it is written in the Google Maps API Tutorial (https://developers.google.com/maps/articles/phpsqlajax_v3?hl=de).

My Problem:

I don't want to load all markers at once, but only those that are between the bounds of my current view.

For that my "phpsqlajax_genxml.php" gets the 4 edges of my current view as GET-variables. That works fine, but I don't know how to handle the javascript-part.

My first try was to insert a handler, that updates the map when the bounds change:

google.maps.event.addListener(map, 'bounds_changed', function() {
  var bounds = map.getBounds();

  var swPoint = bounds.getSouthWest();
  var nePoint = bounds.getNorthEast();

  var swLat = swPoint.lat();
  var swLng = swPoint.lng();
  var neLat = nePoint.lat();
  var neLng = nePoint.lng();

  var qs = '&swLat=' + swLat + '&swLng=' + swLng + '&neLat=' + neLat + '&neLng=' + neLng;
  downloadUrl("./includes/phpsqlajax_genxml.php?sess=<?php print session_id(); 
                  ?>"+qs,function(data){

      var xml = data.responseXML;
      var markers = xml.documentElement.getElementsByTagName("marker");
      for (var i = 0; i < markers.length; i++) {
        var name = markers[i].getAttribute("name");
        var address = markers[i].getAttribute("address");
        var type = markers[i].getAttribute("type");
        var point = new google.maps.LatLng(
            parseFloat(markers[i].getAttribute("lat")),
            parseFloat(markers[i].getAttribute("lng")));
        var html = "<b>" + name + "</b> <br/>" + address;
        var icon = customIcons[type] || {};
        var marker = new google.maps.Marker({
          map: map,
          position: point,
          icon: icon.icon,
          shadow: icon.shadow
        });
        bindInfoWindow(marker, map, infoWindow, html);
      }
  });

But that would update the map every milisecond while I'm scrolling arround the map. How may I update that only every second?

The main Problem is how to insert only the new Caches to the map and delete those that are out of my view. I have no idea how to do that.

Up to now the "DownloadUrl"-function downloads every milisecond every cache in that area and also downloads those that ones I have already downloaded, so you may see there every marker a thousand times.

May someone help me, please? :)