I have been running a three monitor eyefinity setup for over a year now, off a Radeon 6950. Two DVI cables and the third an HDMI cable hooked up through a Displayport adapter.
The monitor hooked up through the displayport adapter is now going black (permanently) after working for a few seconds. If I unplug then replug in the monitor cable, it will come on again for maybe 10 seconds, then go back to black. The monitor stays on much longer than that during the boot up process, but as soon as it hits windows, it goes black after a few seconds.
The monitor does not go into sleep mode, indicating it thinks it still has a good link to the computer.
I've made no recent changes to my hardware or software, and this setup, exactly as it is now, has been working for over a year.
I figure it's a problem with the displayport adapter, but I want to be sure before getting a new one.
Yeah it sounds exactly like a dying DP adapter. I had one (it was Active DP -> DVI) that started doing exactly what you describe. After a couple days of intermittent black screens, it eventually stopped working entirely. Buy a new adapter and you should be good to go.