When the action in the game hots up the avg FPS comes down. That's quite normal for any system as it has to render many particle changes.
I remember how scenes with smoke in them used to cripple high end system years ago. Thought it was great when I got enough power to turn the smoke effects on.
However RAM speed is critical for FX CPU performance, so you might want to contact Gigabyte to see if you can up that a bit.
Also, sounds like you have a freshly installed Windows. Check that you have up to date manufacturer hardware drivers and not only generic Ms ones.