The problem only exists with internet explorer and is version independent. I.e. it occurs with versions 6, 7 and 8. When it all works: 99% of the time, the client browser downloads the elements on the pages. The user interacts with the page and the AJAX POST requests are made to the server. If the user interaction and therefore the AJAX request occur within 5 seconds of another communication with the web server, then the AJAX request may use an existing keepalive connection. This is the normal case when everything works correctly. The problem: Using wireshark, I have found that when the 5 second keepalive expires, a FIN ACK is sent from the web server to the client and the client immediately responds with an ACK. Shortly after (~0.5 seconds), the clients make a POST request using the same connection and then closes the connection with a FIN ACK. No HTTP response is ever sent to the client which makes sense since the server has already closed the connection and cannot send anymore. The request gets logged by apache as a 500 internal server error. The issue is random and seems to be reliant on interaction occurring ~5 seconds after the last communication with the server so that the client will try and use a recently closed connection. Any advice on how to overcome this problem?