seesions problem

I've been using the same code to verify a persons login for more than a year - but I've been having some problems lately - I get past the login, but the session times-out after about 30 seconds or a minute. My new server is using PHP ver 4.3.3. I've also read the docs on "sessions" and I've taken their suggestion and have stopped using session_register("user_id"). I now use $_SESSION["user_id"]). But, like I said after about a short minute, I get the following error message: Notice: Undefined index: user_id in /home/pance/public_html/menu.php on line 13 Has anybody else come accross this problem - and how did you solve it? Pance Stojkovski