Framerate chugging despite computer being perfectly capable

[MadTree]

My rig is as follows;

i5 3570k running at 4.2 ghz
GTX 570 1.25GB VRAM
8GB system RAM

I can't seem to figure out what the problem is, my framerate is horrible no matter what settings I use. Before anyone tells me that my computer is the problem, I am 100% absolutely certain that it is not. How? Well, on occasion, after loading a saved game, alt-tabbing, and alt-tabbing back to the game, I can experience a beautiful thirty seconds or so of gameplay in which the game runs PERFECTLY, at a completely smooth 60 frames per second. It is obvious that my computer can handle this game as I've seen it do so with my own eyes.

However, after about thirty seconds or so of being smooth as butter, my framerate drops again to 30-45 fps, and stays in that range for the remainder of the time. I have tried every combination of settings known to man and I still cannot figure out what is causing my framerate to chug along when my rig has proven itself to be perfectly capable of playing the game at a solid 60 frames per second.

Just to list what I've tried;
1. every single combination of settings in-game

2. forcing vsync/triple buffering in nvidia settings (tried forcing both ON and OFF)

3. changing core affinity and priority in task manager (will occasionally get me a good thirty seconds of 60fps gameplay upon alt-tabbing back in as previously explained, but it never lasts more than roughtly thirty seconds, then it's back to chugging along)

4. revalidating game files in steam, uninstalling/reinstalling

I've also made sure to update, I am running the most recent version as of this morning. Attached is my hpl.log file, I had to compress it because it went over the allotted file size.

If anyone could help me solve this so I can fully enjoy this amazing game, I would love you forever.