The cache in Internet Explorer is not very AJAX friendly for GET/POST commands. You must put in a constantly changing variable in the URL such as date with seconds in Internet Explorer. I noticed this when the refresh of my GET command would only work once in my window.setInterval(.. , .. ) function. However, in Mozilla and Safari, my function was working perfect.
The solution to this is to append the current date and time into the URL of your GET/POST request. Special thanks to this post.
var requeststring = "http://........."+"&requesttime=" + new Date().getTime();
I previously tried setting the cache to expire on the PHP header files, but it has no effect. However, this date/time trick works.