Session variables disappear (some of them only)

Hi ! I have a very weird issue since the last Apache upgrade (-> 2.2.8-r3, a month ago), but I'm not sure it is related (well, I'm pretty sure it's not). Like many people, I've written an application that use PHP session variables, like $_SESSION["my_variable"]. Sometimes (it doesn't happen all the time), _some_ of these variables are not written in the session file and they are lost after a simple header("Location:...); (same domain). The session file is in the right directory (permissions are fine), but some of my variables are missing. The facts : - Apache 2.2.9 + PHP 5.2.6_rc4 running on a Gentoo (up-to-date) - all my scripts begin with session_start(). I've tried to add session_write_close() before every header(Location:...) call, it doesn't help. - I didn't change anything in my program (it has been running just fine for 2 years), it just began to fail from time to time (I would say 10 times a day). There is no hidden unset() function : it would fail for everyone. - these variables are all set correctly, and they don't have reserved names. - only a few variables disappear, but they are always the same ones (could it depend on their position in the session file ?!?) - the session files are very small (max 100ko) - it seems that it doesn't depend on the browser, but IE6 and IE7 seem to be the most affected ones (it may be because my users mostly use these browsers). - I can't reproduce this issue from my local network (any OS/browser - it would be too easy :) - reverting to the previous stable Apache and/or PHP versions doesn't help. - I didn't change any php.ini directive. Any idea ? Thanks ! PS: if you need more details, just ask. The only thing I can't do is pasting the code : the scripts are quite huge.