I have the same issue. FPS drops to <5 when I go close to idle after having throttled up. If I leave the throttle near idle then my FPS evens out, until I throttle up and then re-attempt to go near idle.
My specs: i5-6600k, R9 280, 16Gb DDR4, 240Gb SSD.
My game is a fresh install, latest GPU and audio drivers. I can't find a solution.