Check that your server specifications are well enough. RAM, CPU load etc. Many sites are on shared servers with 100's of other sites on one server. I have had to change web hosts because of this. I am happy with HostGator.
Some wordpress installs run out of available memory because it is set
too low in PHP.
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, change the line in PHP.ini
If your line 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:
php_value memory_limit 64M
3. Try adding this line to your wp-config.php file:
Increasing memory allocated to PHP
4. Talk to your host about their recommended solution.