Problem with Unity 4 and Nvidia GTX 600 series video cards

Hi, We seem to have found a strange issue with our Unity 4 game, which we are unable to solve. A week ago the game ran perfectly (60fps+) on all machines that had the spec to handle it, but recently the game has stopped performing on all GTX 600 series video cards. On 400 and 500 series its still performing the same (60 fps approx.)but on a GTX 670 fps has dropped to 15 fps at any resolution (altering resolution does not effect performance which is strange). We have run the profiler on a 500 series card and compared to a GTX 670. On the 670 card the profiler shows an average frame taking 60ms on the GPU, on a 5-series card it takes roughly 15ms. The draw call count is ~650 this seems to be the same on all 600 series cards. Obviously this delay is causing the issue - is there any reason we should see this behaviour or is this a bug with Unity and the newer NVidia cards?

Regards

Paul