I have theory of Hot NB temps.
The NB (core) voltage should be 1.1v stock. Often times also called chipset voltage. This is a different voltage from CPU/NB voltage and the chipset or core NB voltage will directly effect that heat output you claim.
We can only imagine what the board is posting the core/chipset voltage while perhaps overclocking ram frequencies. Sometimes leaving voltages on auto becomes a bad thing, may be only directly related to 970 chipsets, maybe not. research here would be required.
Another idea is the heat sink it'self. Location, size, even amount of mass in comparison to other heat sinks of more stout boards, it is kind of small.
NB located above a hot back side of the video card also helps to increase temperatures. theory is heat rises. Video cards easily can run 80c, idle 40c or more, depending on make and model.
In my professional opinion, if the heat sink is hot, it's doing it's job, has good contact but poor dissipation from lack of fresh air flow.
Actively cooling hardware is a must in some circumstances. IE: I have a AM2 server board that suffers from heat. Even the bios chip and PCB gets warm/hot.
Determining WHY it gets hot is simple. It's turned on, power is being passed through in the way of voltage wattage and amps if you will.
Oh me oh my, how many dells and HPs have I come across that ran "hot" for years on end. all stock, just runs hot.
If this was mine, I would be sure to manually set NB core/chipset voltage to 1.1v and see if there would be a change in temperature. Physically monitoring it besides watching software that may or may not be accurate.
And yes, you will get many opinions and facts. Only you can decide what's what and make use of the information provided.
GLHF.