Let me ask a few questions:
Did this happen before?
Can you post your entire specs please?
Few things you could try:
Re-install the game.
Re-install your GPU's drivers.
Troubleshoot your hardware, check that your GPU is still working properly, your wires etc.
"Any thoughts about what might be happening here?"
According to tomshardware website, based on that behaviour you described it could be a PSU problem. Your power supply can't handle your system build properly.
If you want to read more about it:
http://www.tomshardware.com/answers/...-randomly.html