That board doesn't have great power delivery, but should be enough for the 870k as long as you aren't massively overvolting it to try to achieve a high overclock. First, try setting your Windows Power Plan to maximum performance, this will force the CPU to run at max clocks at all times, if it's still throttling then it's not a Windows setting issue. After that, if you haven't tried already, reset the BIOS to the factory defaults and test to see what your CPU clock behaviour is in game. If it is still throttling, then there is probably a cooling issue somewhere eg. CPU is overheating, or your case has very poor airflow that the VRMs are overheating despite your CPU's low power draw.