What you are experiencing is what is known as VDroop. That means that when you stress test or put load on your computer, it will actually not feed what you set on the motherboard unless if you tinker with your mobo's LLC settings. Not all motherboards have this option, but your mobo probably has it. LLC stands for load line calibration and you may want to experiment with that to get the voltage that your system needs to get stable.
That being said, I really do not trust the automatic overclocking feature of some motherboards because, if anything, it just feeds your procie may way too much voltage than it ever needs. Overclocking is something that requires a lot of time and testing, but if you can manage, you will have unlocked the full feature of your processor and get to enjoy it for a long time.