Bug? Or are we stupid?

Hi all, First: please also reply to me personally as I am not yet subscribed to this list, but Second: have a weird problem. My Problem is session-related. Have a look at http://www.bioanalytik-muenster.de/a.phps - Nothing special about it's source, just a pure form plus some session and phpinfo() (using trans-sid). If you call the real file: http://www.bioanalytik-muenster.de/a.php and press submit while watching the sessid you will see that it changes the sid - sometimes... After pressing the button two or more times it suddenly saves the id. This is really driving me nuts, but I have two servers where I can reproduce it (second is http://roadrunner.bartels-schoene.de/a.php) - on the second one it does not happen that often (which is another weird fact). Did I do anything wrong? Ba-no.clue.anyomre-lu