"FPS lag" that term makes me cringe so hard. It's lag or FPS drop, not both. They're two completely separate terms, one refers to connection (and more precisely latency) and the other refers to framerate. Please excuse the off topic reply.
In response to the question try rolling back your driver anyway, even if you're on an ATI card. Alternatively you can try updating your drivers. Also validate your game cache. Have you made any changes to your game/system recently? If so undo them and see if that fixes it.
EDIT: Also try using/removing an FPS config you are/aren't using one, try a different DirectX level. Maybe you're using some custom texture or a custom sound that's broken and bugging the game out? Next time it happens, please check the console and see if there are any error messages related to the problem.