Multiple users unable to login

Hi,

I have a wordpress site, for which users must login to access most of the pages.

I seem to be able to login to the site fine, and have given each school using my site a login also.

When more than a couple of people login from the same location, using the same user details, the site becomes unavailable for about 5 minutes.

Currently, I have removed the need to login to my site completely, and the problem seems to go away, so I’m guessing it’s a problem with either the login plugin(s), or with the php that checks whether a user that hasn’t logged in yet is allowed to view a particular page.

Here’s the website:
http://tinyurl.com/n3qqsnb
(I’ve not used the actual url so that it doesn’t come up in search engines).

The plugins I have for logging in are:
Custom Login
Peter’s Login Redirect

And the php that I had to remove from the header to check whether a user can view a page:

global $current_user,$user_ID;

if ( !is_user_logged_in() && !is_admin() && !( is_page(‘wp-login.php’) || is_page(‘register’) || is_page(‘menu’) || is_page(‘running-a-python-program’) || is_page(‘python-introduction’) || is_page(‘python-text’) || is_page(‘python-numbers’) || is_page(‘python-variables’) || is_page(‘simple-gui-programming’) || is_page(‘using-tkinter’) || is_page(‘python-widgets’) || is_page(‘pygame’) || is_page(‘pygame-intro’) || is_page(‘pygame-tilemaps’) || is_page(‘programs’) || is_page(‘useful-stuff’) || is_front_page() ) ) {
wp_redirect( ‘http://usingpython.com/register’, 302 ); exit;
}

…

<?php

if( is_user_logged_in() ) {

if ( isset($current_user) ) {
echo ‘Logged in as: ‘ . $current_user->user_login;
}
echo ‘  //  Logout‘;
}

else {

echo ‘Login‘;
}

?>

Hopefully someone can point me in the right direction to solve this? Please let me know if there is any more information required.

Thanks in advance,
Rik.