404 error

[eluser]Unknown[/eluser]
Hello, I`m using codeigniter with nginx/php-fm.

I saw in the codeigniter error log some 404 errors given on existing pages (working in normal conditions).

I tried a stress test on a static page (.html) no errors occured
I tried a stress test on a php page with phpinfo() no erros

When I try to test some codeinigter links from 1000 requests about 100 are 404


Thanks,
Andrew