I found the free NirSoft BlueScreenView to be incredibly helpful. It shows that the nvhda64v.sys driver is causing problems. My understanding is that the driver is for sending sound through HDMI, but I don't need that. I use a standard stereo plug with my headphones and that works great. My system became stable again after I disabled NVIDIA High Defintion Audio in Device Manager. The driver version 1.3.18.0 that ships with Windows and the 1.3.26.4 that came with latest graphics driver both caused instability. In Device Manager, simply right click on NVIDIA High Defintion Audio and select Disable. If it is disabled, it will have a down arrow overlay:
After I upgraded Windows 8 to Windows 8.1, my system blue screened shortly after plugging in my HDMI cable for my other monitor. The two crashes captured above were shortly after unhibernating. I initially though it was the graphics drivers, so I upgraded those to the latest. They are working great, but I'm not sure it was necessary to upgrade. If you do install them, I recommend just installing the graphics driver.
The upgrade from Windows 8 to Windows 8.1 took about an hour and a half. Windows reboots numerous times during the process. As recommended here, I made sure that Boot Camp was configured to boot into Windows by default. I did not burn a dvd. I simply mounted the iso by double clicking on it and ran the setup.
Hopefully, this saves someone a few hours. Good luck!