Try this
Code:
<script language="JavaScript">
<!--
var r_text = new Array ();
r_text[0] = "IFRAMECODE1";
r_text[1] = "IFRAMECODE2";
r_text[2] = "IFRAMECODE3";
r_text[3] = "IFRAMECODE4";
r_text[4] = "IFRAMECODE5";
r_text[5] = "IFRAMECODE6";
r_text[6] = "IFRAMECODE7";
var i = Math.floor(7*Math.random())
document.write(r_text[i]);
//-->
</script>