Problems with a download center

One of my customers have problems with a download center that I have created. The download seems to work well for most of the files. The problem occurs randomly, and consists in the file download does not start for random files. There is nothing wrong with the files that have been reported. The files are saved in the correct directory with file names similar to the one stored in the database and the files are not corrupted.

There is not a php error occurring and it's not an ivissible session. I have tested it by adding the following code at line 1:

<?php
@session_start();
error_reporting(E_ALL);
ini_set('display_errors','on');
?>

I have checked my php.ini file. Memory_limit is set to 256M and Max_execution_time is set to 60. Those settings should be more than enough, but I have added the following code at line 1 to be sure:

<?php set_time_limit(0); ?>
<?php ini_set('memory_limit', '512M'); ?>

My client is a commercial printer so the files are large, but it does not seem to be any correlation between file size and error situation.

I'm attaching my download file.

You can see an example on live URL here: nedlastinger.php?Kunde=37
In this case, you can't start a download for file number 3. The first two works perfect.

I'm really confused here.

Regards
Rune