I'm having the same problem with the same exact monitor.  I find that when it's stuck you can just unplug the monitor cable, and plug it in again to get back to a working mode.  However there is no option in the monitor's menu to disable this feature!  What's up with that Dell?

This should help: http://forums.us.dell.com/supportforums/board/message?board.id=dim_monitor&message.id=33721 if you have an nVidea or ATI graphics card..... unfortunantly I've got built in Intel graphics....