Try detatching the internet-router from your local network and set it up again.
If that works, you know if the router was the problem.
If it is, that's propably, because there are 2 dhcp-servers. one on the core and one on the router. for diskless md's you definately need the core to be dhcp-server. So you should deactivate dhcp on the router and use the core instead. Assign a second static address on the core that can connect to the router (e.g. 192.168.0.5 if the router is 192.168.0.1) (linux can use multiple ip-addresses on a single nic). If the core has internet connection, every other pc will have as well, because it acts as a forwarder and firewall.