Best way to handle web app connection timeout ?

[jbv]

I've built a client web app that connects to mySQL
on a distant server. Every user has a login & pwd
that are handled by a cgi script on the server.
There's also a 15 min timeout : after 15 min of
inactivity, users need to re-enter their login &pwd.

Everything works fine, except in 1 situation : if
the user's computer freezes and needs reboot, when
he tries to re-log, the cgi script detects that some
activity occured during the past 15 min and refuses
to log. So the user needs to wait several minutes before
he's allowed to relog.
At first I thought I could compare the IP when re-logging
with the IP of the previous activity, but that doesn't work
in 2 situations :
- 2 users working on the same LAN will share the same IP
- if the computer connection is due to a net access problem,
when rebooting and reconnecting, the user might be given a new
IP by his ISP...

So here's another solution I've been thinking of : after logging,
the web app would automatically send a brief authentification
request every 30 or 60 sec. In that case, any gap of more than
60 sec would be considered as a deconnection, and re-log would
be much easier. This method could also be used on a website
with a hidden frame configured as push.
The drawback of this method is that it increases the server activity...

I didn't mention it before, but it is obvious that I don't want to
use any form of cookie...

So I'd like to know what do you guys think of this, and if there's
any better solution available...

Thanks in advance,
JB