Donno if this relates but i had a similar fps drop today.
I always have 60fps, did not change my graphical settings.
When i started a game i felt like the game was lagging so i thought it was the server.
After changing a server or 2 i noticed my fps was dropping to 20 ?! + not getting higher then 30!
I played again later on the day and everything was fine..
There were no programs running on the background, in fact i had more programs running later in the day when the fps were fine..
I also have this in zombie mode, sometimes my framerate is horrible..