Try changing it in the nvidia control panel to force it to always use the 670. I actually have the same setup but haven't seen this problem as far as I know, I only lag with huge ships with lots of parts. Manage 3d settings-> global settings -> preferred graphics processor. I had to manually add it in the program settings tab I am guessing you have done this hence the gpu switching. Just curious if it's forced if it would work then, a pain yes but then you would be able to play at least. Just make sure to turn it off or you will get crap battery life.
