Captive Portal

Hello Listreaders,
I make the following observation:
In case 30 - to 40 MSN users are using M0N0wall (1.231) trough the captive portal, users complain about  "teminated Sessions"

"relevant" Settings:
TCP idle timeout = default
Firewall states = default
Traffic shaping off

Hints how to track effect this would be very appreciated.
(do we run into a NAT Table limit?)

Thank you for your tips in advance.

Jakob