I have a 2.66 GHz Quad-Core (MacPro4,1) mac pro. Recently I tried to upgrade this machine by replacing w3520 with w3580. After replacing the CPU, I boot my machine and then it boot o.k. I checked processor from about this mac and then the machine suddenly froze without doing anything. After reboot the machine, I got cycling "kernel Panic". At this time, my OS was Mavericks. I tried other OS such as 10.8, 10.7, 10.6. I got kernel panic from both 10.8 and 10.7. But I boot successfully with 10.6. Since I don't have any idea how to fix this problem, I changed w3580 with the original w3520. After re-boot, I received the same kernel panic from 10.9, 10.8, and 10.7. I can only boot my mac with 10.6 with w3520 or with w3580. I am stuck now and I don't understand why I am having the same issue with the original w3520. I try the safe boot with shift key and it worked with Mavericks although everything is extremely slow. Can anyone shed light on this problem?