I've got 3 monitors fed by an HD 6950 2gb. I've got Catalyst 13.1 installed and I'm running win7 64bit. My left monitor is connected to a miniDVI which runs through adapters into a VGA connector into the monitor (these LG E2242C monitors only accept VGA for some reason).
My middle monitor is an ASUS which is connected with DV-I and the right monitor is also an LG which is DV-I to VGA.
Still with me?
The problem is that the left monitor, the only one using a mini-displayport, acts strange. When I boot my PC, the BIOS startup appears on it, and it'll work fine, with a 50/50 chance of displaying my desktop in Windows after booting. If my PC goes in to sleep mode or turns the monitors off, again there's a 50/50 chance of it working fine again upon waking the PC. When the PC goes in to sleep mode, I can hear the "hardware connected / disconnected" sound playing quite frequently.
Now the oddball to this is that the monitor has a permanent signal. When these monitors lose signal, the power light blinks as any monitor will, but this doesn't happen in this scenario. The light is permanently on, whether the PC is outputting a signal to the other two monitors or not. This is also evident in that the backlight is permanently on too, despite that the screen is black. It's rather frustrating and I've updated both Catalyst and Windows comprehensively but the problem still persists. I'm thinking it's a dodgy driver or something but I can't for the life of me figure it out.
Have you tried rotating the cables to ensure it isn't a cabling issue?
Also try switching outputs on the card if possible (i.e. plug monitor 1's cable where monitor 2 is currently plugged in and vice-versa) so you can determine if it is a monitor, cable, or card/driver issue.