I have been having similiar issues with my install and originally thought it may have been due to a conflict with my modem/router however my setup is in accordance with recommended setup on wiki. It now seems that the internal Eth1 network seems fine after reboot but only for short perod of time after that webadmin is unable to be accessed from any device connected to Eth1 switch nor access internet.
Not sure if it is related but in this post http://forum.linuxmce.org/index.php?topic=10365.0
the DCERouter kept crashing after 14minutes. Anybody know if the issues are related and if the fix used is applicable. I will give it a go when i get home.