Windows 7 Black Screen

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.

It's definitely the 5870 overheating, I just can't figure out if the issue is the card or the PCI port.

If it's the card, I'm not sure why moving ports has fixed it, and I don't understand why the card fails differently depending on which OS is running?

Thought I'd post back with a small update. My 5870 black-screened me shortly after booting into Windows 7 again, though it was still in PCI Port 2. Took it out, reseated it back in Port 1, reset SMC and PRAM, and it came up fine in OS X, booted back into Windows 7 while watching the GPU fan and everything seems back to normal again.

This obviously rules out the PCI Port as the culprit, as I'd not used Port 2 since I bought the machine in 2010, and I doubt it's a software issue as there can be issues under Windows or OS X. What I don't get is why user OS X it's really rare, and under Windows 7 it's fairly likely?

I did install the latest drivers for an AMD 5xxx series card from the AMD site; I can't see how that's the issue though because as far as I'm aware Apple GPUs are reference models without modification, so the drivers from AMDs site should work perfectly.

I reckon it's a dodgy fan. I've read about people getting self-service graphics card replacements directly from Apple, is that true?

Brief update, turned out it was the fan on the 5870. Apple swapped it out under warranty and it's all back to normal