My experience with frame rates has been very schizophrenic to date, and I think a lot of it depends on the server you're playing on, as opposed to any lack of power in your rig. One thing I have noticed is that I can be running at a smooth 60FPS, and then it'll drop down to 20. Sure, I think, it might be a lag spike, or a heap of zombies/players around, it won't last. But it will. If the FPS drops, for whatever reason this is, it never returns to what it was previously, even when I'm in the middle of wilderness with nary a zombie in sight. So it might be a memory issue, or maybe I'm still simulating something on my end when I shouldn't be. Either way, I'm pretty sure it's an optimisation issue as opposed to an issue with your computer. I've read in chat other people have had their FPS tank at around the same time mine has, though that might be coincidence.