I made an earlier thread about connection issues I where having due to my router firewall: http://forums.spiralknights.com/en/node/9311.
I have since then solved this problem (turns out I hadn't properly assigned my IP for the port forwarding) only to be presented with another one: constant disconnects. Essentially, when port forwarding is enabled, the game disconnects every other minute. If it's not, I get my old problem instead.
Like before, I know this is network-related. The game works fine on other networks.
My router is a D-Link DIR-600. It has the latest Firmware (2.03).
According to pingtest.net, I have a Grade B "Very Good" connection with servers in Washington.
Servers in Washington state? The Three Rings servers are probably hosted somewhere closer to the east coast...Or so I think.
Do you suffer this problem when your computer is connected directly to the modem, no router at all? If so, it may be a general problem with internet service in your area or a problem with your OS setup.
As Dogrock suggested in your first thread, DD-WRT firmware may help you with your problem, if it is indeed just a firmware issue.