Last June I upgraded an 8-core 2.93 GHz MP4,1 to a 12-core 3.33 GHz 2x X5680 MP5,1. All went well at that time. It has been running latest Mavericks OS since June 2014. In addition to adding/installing the 2x Hex core X5680s the following items were also installed 1) OWC Mercury Accelsior PCIe R0 480GB for boot volume 2) 64GB PC10600 (8GBx8) MacPro DDR3 ECC (1333 MHz) 3) 2x 240GB OWC Mercury Electra 3G SSD 7MM in bays 1 & 2 for fast Cache 4) K5000 for Mac The Geekbench 3 (64-bit) scores for the MP4,1 were Before the upgrade Single core = 2347 Multi core = 18427 (8 cores @ 2.93 GHz) The Geekbench 3 (64-bit) scores for the MP5,1 were After the upgrade Single = 2714 Multi core = 30745 (12 cores @ 3.33 GHz) I ran i/o tests to the PCIe 480GB SSD and recorded 680 MB/s I ran i/o tests to the 3G RAID-0 240GB SSDs and recorded 520 MB/s Two days (Jan 15, 2015) ago this upgraded MP started to panic... see attached picture as an example. The machine simply froze up, crashed then just restarted itself over and over (for 30 minutes) and kept getting these kernel panics. The number at the beginning kept changing each time it crashed (cpu 12 caller, CPU 8 caller, etc). It won't boot up past this panic screen. Tried reseting NVRAM PRAM, safe boot, and pulled out all the memory and third party peripherals out, cleaned it out, reseated everything, and left everything unplugged but monitor and keyboard/mouse. Tried also just having a few sticks of ram rather than 8. Nothing made a difference. Given that the panics have been indicating CPU 12, CPU 8, etc I'm thinking maybe CPU B is at fault on the assumption that CPUs 7 through 12 are core numbers and that this signifies CPU B. The X5680s were purchased from ITCreations in CA. I've contacted them and they are willing to work with me on a free X5680 replacement if indeed I can determine if the issue is just one of the X5680s. They advised to move the CPU B to the CPU A position and test the system running CPU A only to see if the presumed CPU B is at fault. I'm looking for some advice from others here. Looking at the attached picture what would you determine the cause might be for the panics ? I'm assuming the issue is related to one or both of the Processors and that the additional components added (RAM, SSDs, K5000) have nothing to do with the issue. Thanks for any feedback and/or insights about this.