Fx-8320_throttling

Hellow there, I've built a pc ,and I experience a throttling issue with the cpu. 

I have overclocked my cpu at 4.7GHZ ,at 1.488v have disabled turbo and some other bios features.  I do a prime test , none of the cores is failing but I'm monitoring the cpu freq with cpuid and task manager at the same time ,to a have a more valid result. At the first minute the cpu runs with no issues , after that the cpu starts throttling at 3.5ghz. One minute later the cpu throttles back to 1.6ghz , again 3.5 , and again 4.7, it flactuates there , and changes freq almost every 5 seconds. I'm checking cpu and mobo temps with cpu hw monitor,and the cpu isn't getting more than 53 , so does tmpin1,2,3. Checking some forums, I have discovered that is a mobo issue or maybe the mobo isn't cooled enough. After all this mess with overclocking I decided to reset everything to stock (3.5ghz, bios features and more). Loades defaults, and ran prime again. I experience the same throttling againg . I have to say though that at 4.7ghz , I tested prime with 3 cores only ,and it doesn't throttle. Generally I don't see any performance hit , computer never crashes , neither in games neither on simple usage, and gaming performance is perfect .

My point is that: Is it possible be a  software bug or my mobo overheats and maybe I need to cool vrms more or change mobo?

Specs :

Mobo: Gigabyte 970a-ud3 rev3.0

cpu :fx-8320

gpu: sapphire radeon hd7850 2gb

psu: corsair cx750m

cpu cooler: scythe scnj-3100 ninja3 revb.

I have installed one fan hitting the gpu and one more on the cpu cooler (besides the stock one) for throwing the hot air on the back.

I can almost guarantee it is that motherboard. The 970 boards aren't designed really.for the 8 Cores especially not at that voltage. 

Plus Prime95 really isn't a very good test and may be causing some problems.