session id changing all the time on some pc's

Hello, I have a strange problem. For example i have a script: <?php session_start(); echo session_id(); ?> Once it started a session the session_id should never change. It works on my localhost with default php configuration. I'm using mozilla firefox and IE. I put this script on some other server on the net. It works just fine too with both browsers. But i also put it on one other server and while you are you using IE the session_id changes all the time. With mozilla firefox everything just works fine. On all servers. I looked at server configurations but i couldn't find anything usefull. I tried to change IE settings to accept all cookies but nothing changed. Maybe someone knows where's the problem.. Thanks Bye