Can you try setting the memory limit for PHP up? See http://ez.no/developer/ez_publish_3/documentation/ez_publish_3/typical_problems_and_solutions/not_enough_php_memory
Could be a timeout problem (Apache/PHP needs to long to process the page). Use PHP as an Apache module, if you can (will be faster) or try to increase the max. execution time for php scripts.
I found some errors in my Apache error.log file, like:
Premature end of script headers: e:/php/php.exe
File does not exist: e:/apache_group/apache/htdocs/ez_publish File does not exist: e:/apache_group/apache/htdocs/ezpublish/design/admin/stylesheets/debug.css
One time i receive this error: FATAL: erealloc(): Unable to allocate 1634623366 bytes