Session variable not persisting

Hi all, 

I'll be swift in my explanation as I can't find in any way the source of the problem; it seems to be installation-related. 

At execution, sometimes (randomly it seems at first), variable session aren't properly stored. 

=====================

First things first, about installation : 

uname -a -m -p yields :

Linux cl-t180-253cl.privatedns.com 2.6.18-308.1.1.el5 #1 SMP Wed Mar 7 04:16:51 EST 2012 x86_64 x86_64 x86_64 GNU/Linux

php -v yields :

PHP 5.3.3 (cli) (built: Jun 27 2012 12:25:48) 
Copyright (c) 1997-2010 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies

=====================

To make sure it is not caused by any other part of my scripts, I just created two scripts that have replicated the problem :

SCRIPT #1 [ping.php] ################

<?php

session_start();

if(count($_SESSION['in'])==0){
    $_SESSION['in'][0]='0-initiate';
} else {
    $_SESSION['in'][] = count($_SESSION['in']).$_GET['push'];
}

if(count($_SESSION['in'])<1000){
    $rand = rand(0, 1000000);
    ?>
        <meta http-equiv="refresh" content="0;URL='pong.php?push=<?php echo $rand; ?>'">
    <?php
} else {
    var_dump($_SESSION);
}

?>

SCRIPT #2 [pong.php] ################ 

<?php

session_start();

if(count($_SESSION['in'])==0){
    $_SESSION['in'][0]='0-initiate';
} else {
    $_SESSION['in'][] = count($_SESSION['in']).$_GET['push'];
}

if(count($_SESSION['in'])<1000){
    $rand = rand(0, 1000000);
    ?>
        <meta http-equiv="refresh" content="0;URL='ping.php?push=<?php echo $rand; ?>'">
    <?php
} else {
    var_dump($_SESSION);
}

?>

##################################

In fact, when I run this on my server I randomly have empty entries in the $_SESSION array. Any thoughts on this? Is this a known bug (haven't found it…)

Thanks!