This could also be an issue of too little memory allocated to php. If you have root privileges on your server, edit the php.ini file and either add or append the following line.
php_value memory_limit "16M"
If you don't have access to your root, you can ask your host to increase the memory size or try adding it to a .htaccess file to your forum root with the above line, without the quotes around 16M. The other alternative is to add this line to your common.php file
ini_set('memory_limit','16M')
after the lines
if ( !defined('IN_PHPBB') ){ die("Hacking attempt");}