I do the "not logged in" test now regularly when I get a 504 with my autologon browser. Today there was a clear difference, the session with autologon getting 504's and the session without autologon showing the forum.
That sounds like not-logged-in pages might be cacheable, but when you�re logged in, it has to hit the database for the extra information you get. If that database is overloaded (or crashes), that could explain the regular dead forums.
I hope no one has to restore the database manually every time.