This is really strange. I think I finally got rid of the DXGI ERROR REMOVED issue (fingers crossed) by setting VSYNC in the nVIDIA control panel and disabling it in game, but have another strange issue. I can play the game perfectly on EXTRA HIGH for 40 - 45 minutes and then I get extremely bad stutter -- in fact it locks up and if I can manage to exit the game my mouse cursor has disappeared. Thinking it was a heat issue I ran a GPU stress test for about 15 minutes and the temperature got up to 86 C fairly slowly and then stayed there for the rest of the test.
So it may not be a driver issue (or else why could I play for so long). Any ideas?
Thanks
Alan

Intel i7 3770 3.4 CPU and GTX 680 GPU