This is usually an indication that your php memory limit or max_execution_time is set too low. See below:
You can define this in your php.ini file (usually found in /etc on linux systems check your documentation for windows servers). Look through he file for a line like this:
memory_limit = 128M
Keep in mind the 128M is probably different. That’s the amount of memory that php is allowed to consume. If you have 10 plugins and combined they consume more memory than you have allocated, you’re going to have problems. You can assign more by increasing this value. (my personal sites have 256M allowed, but this is for a pretty big site with a substantial number of hits and plugins). Make sure and restart httpd (apache) after making changes here.
Let5 me know if this fixes it.
tim