Win 7, 8GB RAM, Core i5 @ 3.5 GHz, GTX670
*Points at the GTX670*
There is the problem. nVidia cards and drivers are buggy beyond belief.
In my experience it has been very difficult to get things to run properly using basic DirectX feature. AMD aren't the best cards, but for the basic stuff they tend to just work.
My friends who professionally write games have the same experience. They tell me they program the game, then spend at least a month trying to get it to not crash and run smoothly on nVidia cards.
Intel graphics they like because they have the most stable drivers, but the low performance they don't like.
Of course Star Citizen Alpha 2.0 isn't the most stable thing in the world, but I can usually play for a while without it crashing. Alpha 2.1 I experience the performance problem they mentioned pretty quickly. For me it seems to happen when trying to fly the Freelancer.