I recently started getting 502 and 504 errors while trying to read fimfiction. Somebody added the fimfiction servers to one of PeerBlock's block lists. If you use PeerBlock, open the PeerBlock user interface, the one that shows IP addresses being blocked. (Set it to not show connections that are not blocked.) Close your browser, then open it with nothing but fimfiction (this is not essential, but prevents you from allowing other servers that are trying to download pop-ups or worse). Watch for the server that your web browser (your machine) is trying to open an http connection (port 80) to every few minutes when you have fimfiction in your browser, and allow that address permanently.