var iframes = document.getElementsByTagName('iframe');
for(var i = 0;i < iframes.length;i++){
   if(iframes[i].src.indexOf('?') > -1){
    iframes[i].src = iframes[i].src + "&extra=" + Math.floor(Math.random()*10000);
   }
   else{
    iframes[i].src = iframes[i].src + "?extra=" + Math.floor(Math.random()*10000);
   }
}