I have been having similar issues, only mine predominantly happens whenever I go to make a new firewall rule, it will just sit there until it times out, refreshing doesn't help either. This happens both when on a remote system accessing the web admin and when local on the core itself. I have also seen random issues with traffic will just stop passing to port 80 outside my network, but internal will remain intact. Flaky NIC? I really don't know where to start troubleshooting this issue, I've verified that all my network settings are correct, and really think it's a software problem since it also happens when local on the core which is not even passing traffic through the nic, it is hitting the loopback address...