Powercolor HD 7870XT PCS 2GB Tahiti LE Crossfire problems

Ok, this has got me losing my mind. I have two brand-new Powercolor HD 7870XT 2GB cards arranged in Crossfire on my board, replacing my venerable XFX HD 4870 1GB cards that were there before. As soon as I had them in, the "fun" began. I couldn't run any kind of benchmark without freezing or crashing, I couldn't uninstall the driver (13-9, the newest one), when I reset my computer, Windows shuts down properly but then the computer just sits there. In addition to that, when I shut down, again, Windows shuts down properly but the computer won't turn off. These symptoms only exist when the two cards are installed (crossfire or not). I re-installed windows, I tried three different versions of Catalyst (all have more or less the same effect), I have basically been losing my hair over this because the only symptom I come across is freezing or monitor-off crashes. I've been running Crossfire for years and I know what I'm doing (at least I thought I did until now...lol). Hell, even trying to load GPU-Z results in a blackout crash. I had finally had enough and popped my BIOS battery in an attempt to completely reset everything. The cards actually worked for a bit then, I ran 3DMark Vantage, got a score of P30045 and figured that everything was hunkey-dorey. Then I did a Windows 7 update and oh boy, here we go again! The update removed Catalyst (somehow) and when I re-installed it, it all started happening again. It also crashes if I try to uninstall Catalyst or install another version. Heck, it even crashes if I just try to disable Crossfire! Does anyone have any idea what in blue blazes is going on? Here are my system specs:

CPU: AMD FX-8350 4Ghz (no overclock)
RAM: 16GB (4x4GB) UMAX Cetus DDR3-1333
Motherboard: Gigabyte GA-990FXA-UD3
PSU: OCZ Z-Series 1000W Modular 80-Plus Gold Certified, single +12V rail rated at 83A.
Windows Version: Windows 7 Ultimate x64
System Drive: OCZ Vertex3 60GB SATA3 SSD
BIOS Version: F9 (newest non-beta BIOS version)
Catalyst Version: 13-9 (newest non-beta display driver)


To make things even more complicated, both cards work flawlessly with the drivers if they are alone. This only seems to happen when there are two cards installed. Does anyone have any ideas? I'm at my wit's end with this thing. I'm usually able to overcome problems like this on my own but I have tried everything I could think of (including changing the Crossfire connector) and I am just losing it now. Thanks to anyone who can help me.