I have installed a new 2820N for a customer in the last week. It has the latest 3.3.3 firmware. Outgoing internet connections are fine and it is possible to connect consistently from offsite to the management interface of the 2820 without losing the connection.
However, four VPN connections using L2TP with pre-shared key have been set up. I can connect consistently using any of the four usernames but the line drops after 1-2 minutes, whether actively being used or not. At the remote end, this is detected and triggers a redial. All remotes are XP-Pro SP3.
There is another one with another customer which stays connected with no dropouts. I can see no difference in the setup.