The game earlier worked perfect and with FPs around 60, but today, I logged in to find the game's FPs lowering to 4-10 and I was getting frequent crashes.

I have updated my graphics, but there was nothing different.

its happening to me too..its weird at my whole pc`s and laptops isnt lagging only sometimes between 2-16 FPS.....it was over 27-34 FPS :S

Is your computer overheating? How much RAM usage was before and now? Did you turn on any shader options that could have effected the FPS? What's your Graphics Card?

Try shutting your PC for a while and turning on again.
Mostly works.


PS.
This is my 100 th comment