I've got a system with an AMD Radeon HD 6670 PCI-E card and an NVIDIA GeForce 7025 / NVIDIA nForce integrated desktop motherboard graphics.
I have a Dell 19" flat-screen monitor plugged into the motherboards VGA port.
I have a Samsung 22" flat-screen monitor plugged into the VGA port on the Radeon.
I have an Apple Cinema 20" wide screen (M9177LL/A) plugged into the DVI port on the Radeon.
Windows 7 (64-bit) works great with this configuration.
Windows 8 (64-bit) seems to work fine for awhile and then the cinema display will turn black and the monitor will be unresponsive. During this time, the white-light near the power button will stay on and the power button itself won't work.
If I disabled it as third-monitor in the windows screen resolution preferences and then enable it again, it still doesn't work.
If I reboot the system the monitor still doesn't work.
Ultimately, I have to unplug the display from the power strip, plug it back in, and then disable/enable the monitor again in Win 8 to get it to work.
I've tried using the latest nvidia and AMD drivers (including beta drivers) but that doesn't make a difference. It's as if something in Windows 8 forces the monitor to enter this unresponsive state.
Any ideas would could be happening here? I don't think it's strictly a hardware issue itself because, it works flawlessly in Windows 7 (dual-boot on the same system).