var content_so = document.getElementsByName("aa");
var max = content_so.length;

var i = 0;
var curr = content_so[i];
InsertContent();

function InsertContent() {
curr.style.display = "";
i++;
setTimeout("RemoveContent()", 5000);
}

function RemoveContent() {
curr.style.display = "none";
if(i>=max){i=0;}
curr = content_so[i];
InsertContent();
}