That happens when there's a high load on the server. This page will let you know when it's a good time to install and will provde you with a link to Fantastico. If you see the server load is to high (above 5), try again in 30 minutes or so.
http://byrondallas.heliohost.org/server_load.php
btw, you can also see the server load at your cpanel Service Status.