Some WordPress installs run out of available PHP memory because the memory limit is set too low.
Try this plugin to see the PHP memory limit and usage in admin dashboard:
If the memory limit is 32M or less, try increasing it:
Memory usage is dynamic, meaning that when you are uploading images, the memory usage can easily double temporarily.
Try one of these methods for increasing the amount of memory a
PHP script may consume (most people just do #3)
1. If you have access to your PHP.ini file, edit the PHP.ini
If the memory_limit shows 32M or less, try increasing it:
memory_limit = 64M;
2. If you don't have access to PHP.ini try adding this to an .htaccess file in the root directory of your site:
php_value memory_limit 64M
3. Try adding this line to your wp-config.php file:
4. Talk to your web host about their recommended solution.
If your web host will not allow more than 32M memory limit,
I would get a different host.
My web host allows me 128M memory limit.
(also make sure your server is PHP5, not PHP4)
PHP5 is more secure and more reliable. PHP4 is discontinued.