When I've tried to upgrade to either Mac OS 10.8.5 or to Mavericks, my Mac Pro freezes, sometimes instantly or after a while. I have a Mac Pro 4,1 which has been flashed to 5,1, with 32GB RAM, and I have a GTX 680 Mac edition and GT 120 installed also. I have tried with either just the GTX 680 or the GT 120 installed with the same results. This evening I realised I still had my old hardware test disc, the result was it found an error with the videocontroller. I repeated the test with both GTX, and GT installed and then with just one installed, with the same result. Am I right in thinking the fault is due to the logic board?