I solved this by changing power setting in the Nvidia control panel, stock setting is Adaptive, I set it to "Prefer Full Power". No crashes since!.
I did change the card though.. just playing WoW, it would it 89c and the fan would ramp up.. I am getting cranky in my old age, the noise was annoying. Picked up a good deal on a GTX 670.. this card hasn't gone higher than 68c and is very silent.
iMac Retina - 32GB- 1 TB SSD - M295X