What level did you play and for how long? I have played the beginning of "Unearthing the Sun" for 300 seconds multiple times the same way and average framerate was between 47-53 doing the same stuff everytime and using the same weapons. It sounds a bit odd that your runs always just differed by 1 FPS around the average.
I played the whole game and i think level 6 is the best to see if a gfx driver sucks or not as the framerate drops at the beginning already. Using an indoor level like 2 is completely stupid as there the framerate is not below 25-30 fps even with slower cards.