I've been playing this for a few months now and all of a sudden, since yesterday, when I am playing I will get a massive drop in FPS to the point where I can't even control the mouse steadily. The drops are intermittent and last for a random period of time; sometimes only a few seconds, others it will stay until I close the game.

Was there some kind of update? I haven't changed anything on my system, not even installed/uninstalled anything, so I have no frame of reference for a possible cause.

It's probably just a normal lag. Everyone once in a while your game is going to lag. It could have possibly been a flame that a player was wearing, a simple Internet connection problem, or something you may have downloaded that made your computer run slow. The possibilities are endless. If it continues I'd suggest read this below.

It is definitely not lag, it is my FPS, as the FPS counter goes from 65 to 4 and has absolutely no effect on the other player. I get this drop even in LAN games, where the other computer gets no issues while mine is seemingly struggling to keep up.

As I said in my first post here, I really have no idea why this has started all of a sudden, when I have been playing for months without issue.