Well, USB pulls from the 5V rail, not the 12V, as does the hard drive. The m2 is only rated to output 40W (8A) on the 5V rail. That motherboard also draws most of its current from the 5V rail. As does the PCI slot. I'd say there's an excellent chance you're overloading the 5V rail on the m2-atx.
Of the USB devices you have listed, the wi-fi device consumes the most power. Try unplugging it, see it you get stability back. If so, get a powered USB hub, and a 5V point-of-load power supply for it.