I have 3 Radeon cards hooked to 6 monitors and the 6th monitor blanks out with a black screen after some reboots. Even on different interfaces. Windows 7 X64 says it is there and the mouse pointer disappears where it should be and it shows it is receiving a signal.
I have had the same problem with earlier 3 card combinations with 6 monitors and earlier drivers but it is getting worse now.
I found I can get the monitor back if I hibernate the system.