Apache times out an resets the session.
Configure Apache:
#
Timeout: The number of seconds before receives and sends time out.
#
Timeout 600
OR
Refresh the page once in a while... (<10 Min):
<meta http-equiv="refresh" content="300; URL=index.php>">
300=5 Min.
Netscape needs this somewhere in the <HEAD> </HEAD>-Section
IE does it, no matter "where it is"