Fatal error: Out of memory

While browsing/admining my drupal website I and the other users on the site occasionally get an error message like this instead of whatever page was expected:

Fatal error: Out of memory (allocated 11272192) (tried to allocate 19456 bytes) in /home/outaouai/public_html/members/drupal/modules/views/modules/views_user.inc on line 325

or

"Fatal error: Out of memory (allocated 2359296) (tried to allocate 19456 bytes) in /home/outaouai/public_html/members/drupal/modules/calendar/calendar_ical.module on line 188"

The troublesome module varies as do the numbers.

Reloading solves the problem, but I'd rather it weren't there.

Some server info:
Operating system Linux
Service Status Click to View
Kernel version 2.6.9-42.0.10.ELsmp
Machine Type i686
Apache version 1.3.37 (Unix)
PHP version 5.2.3
MySQL version 5.0.27-standard

And drupal is v 5.1

I am not sure if the problem is with drupal or my webserver. Does anyone have any insight about what might be going wrong?

Cheers!