The yum-fastestmirror plugin is great for server admins. It goes out and attempts to determine which mirror would be faster to download from and automatically selects that mirror. The plugin works well for the most part but every once in a while it can choose a mirror that is slow. If you want to reset the plugin so that it will retest the mirrors all you have to do is the following:
rm -f /var/cache/yum/timedhosts.txt yum install <package>