I have a MacPro5,1 running a Xeon W3670 3.2Ghz, 8Gb RAM, 6 HDDs and an Apple AMD 5870. I'd been having a couple of weird issues with my Mac Pro recently, mainly while running Windows 7. I would get black screens randomly, sometimes after logging in, sometimes as soon as I booted the OS, sometimes even booting the installation media. The issues initially were inconsistent, but became constant more recently. In OS X, this never happened, though on rare occasion the 5870's fan would kick up to full out of nowhere and the temperatures hit mid-90s (Celsius) , generally while playing back video (VLC / Quicktime / iTunes), or the entire UI would lock up (though sound continued). After ripping my system apart and troubleshooting, it seems the fan wasn't spinning when booting into Windows 7 (and I guess, occasionally in OS X). After doing all the usual SMC/PRAM reset shenanigans, I removed the 5870 from PCI-E port 1, cleaned it, and re-seated it back into port 2 (The other 16x PCI port), and things seem back to normal. I guess my question is, would this be a fault with the GPU, the Logic Board, or a combination of the two? These issues began to occur before I'd swapped the CPU from a W3530 to a W3670, so I doubt that has anything to do with it.