Permission issue – admin-ajax.php

I’ve been getting the following error even after several fresh installs. I get this when i try to add or delete anyting in the system: ‘You don’t have permission to access /wp-admin/admin-ajax.php on this server.’

My current access to the admin directory is 755 and access to admin-ajax.php (and all files in the directory) is 644. Any ideas? PLEASE HELP!!