xeon e5 2640v3

xeon e5-2640v3 is unlocked turbo boost to 3.4ghz, but after few minutes of gameplay it dropps to 1197Mhz and gets stuck on this low clocks until i change resolution (then change back). but after a few minutes it dropps again. any clue? the current undervolt is 50/50 and mobo is a Huananzhi Qd4...