Fatal error, out of memory problem

Hi! We're having problems in our server/Magento.
We've read and tried to fix this very long but without any luck.
Our php.ini looks like this, any idea someone?

Problems can be for some clients to get in on site, while some at the same time can get in.

; This file is for CGI/FastCGI installations.
; Try copying it to php5.ini, if it doesn't work

; adjust memory limit

memory_limit = 512M

max_execution_time = 18000

; disable magic quotes for php request vars

magic_quotes_gpc = off

; disable automatic session start
; before autoload was initialized

flag session.auto_start = off

; enable resulting html compression

zlib.output_compression = off

; disable user agent verification to not break multiple image upload

suhosin.session.cryptua = off

; turn off compatibility with PHP4 when dealing with objects

zend.ze1_compatibility_mode = off

; PHP for some reason ignores this setting in system php.ini
; and disables mcrypt if this line is missing in local php.ini

extension=mcrypt.so

apc.filters = "-*test\.php"

post_max_size = 30M