Hi,
If you have installed eBox from the Ubuntu repository, you will be running eBox 0.11.99 and it had a bug in the module network if you installed eBox and you changed from a dhcp configured interface to a static interface.
You can either updagrade your installation using the packages from our repository, or easily workaround the problem by editing /etc/network/interfaces and removing the dhcp interface.
The problem with that version is that when the machine boots, the system starts the network based on what there's in /etc/network/interfaces. When ebox-network starts, it runs /etc/init.d/network stop and eBox takes care of configuring the interfaces according to your configuration. /etc/init.d/network stop should stop the dhclient process, but sometimes it doesn't and that's why you could experiencing some issues.