When unlocking a Windows 7 computer the screen briefly (less than a second) goes blank before showing the screen. Abound once a month (but sometimes within a few days) when I unlock my computer the screen doesn't come back from this brief blackout and stays black. Sometimes after five minutes or so the display will come back. Other times it has been blank for over 20 minutes, so I give up and restart the computer.

• It seems to happen more often when the computer has been locked for a longer period of time - I lock my computer several times a day, but the problem most happens when I come in at the start of the day.
• I have updated my video card and monitor drivers.
• I have two monitors driven by the same graphics card.
• The computer is a Dell Optiplex 740.
• When the problem occurs both monitors have a green light on to indicate that they are receiving signal.
• I've tried unplugging the monitors from the video card and turning the monitors on and off.
• The screen saver is set to one that is not blank screen.
• The Windows power settings are set to never turn off the display.
• When the problem occurs there is no significant disk activity occurring.
• When the problem occurs I can connect over the network to the hard drive on my computer.
• When the problem occurs I can't connect over the network with a VNC connection. The VNC client doesn't give an error, but also won't show the screen. The task actually seems to hang as I can see the task, but there is no window for it.
• This problem occurred when I was running a pre-release version of Windows 7. The hard drive was formatted for release version and the problem still occurs.
• I've been stopping some of my always running programs to see if one of them may be the culprit, but given the span between failures that will take some time to find the problem if it will even help at all.
• Some programs I have always running include IE, Firefox, Outlook, Evernote, Kana Reminder, Ditto, Macro Express, Timesnapper, and DisplayFusion.

Any ideas appreciated.