I've had a similar problem and it's all to do with time.
What is happening is it is erasing those logged in when you view it as it believes they have been inactive for more than 10 minutes. You probably have a positive time zone offset.
Anyway the fix I used was this.
Delete line 24 and 25 from online.php and replace with
$Outdated = time() - 600;
Now doubt Scream will come up with a more elegant solution, but the problem is all caused by time zones.
Dan
