During testing of the lag, it seemed unlikely that any plugins were the cause.
Specifically, player connect (when you see "Player X has joined the game") is not hooked at all by any of our plugins, although that seems to be when the lag usually occurs (I also noticed it happening on player leave as well at seemingly random intervals).
It is definitely far worse on the US servers than any of the others, so at this point our best guess is that it's hardware-related. The next step will be to either try another host, or move a notch up on the specs with the current host and see if that helps at all, though I'm not sure when we'll be able to go through with that.