Late 2009 iMac display goes black, display does not come back

Hi there, for several years now I have had a very frustrating problem with my late 2009 iMac 27" 3.06Mhz display. This computer is on 24 hours and never sleeps since it is hooked to some weather hardware that I have running. I also use this iMac as a secondary display with mid 2012 macbook pro when I am working at home via a display port cable. However, when I lose power at home or the iMac has to be shutdown abruptly, the display starts misbehaving: When the display goes to sleep and/or the computer goes to sleep (I will make the distinction whenever appropriate between "display sleep" versus "computer sleep"), the display will not come back on and remains black but the iMac itself is alive and well (I know this because I can perform remote management and screen sharing with it no problem.) So the computer only comes back on, NOT the display. It does not matter if the iMac is flying solo or I am using it as a secondary display, the same problem happens with either situation.

What fixes it? I have no idea but what I usually try is: an SMC reset by shutting down the computer and waiting anywhere from 15 seconds to overnight, a PRAM reset by the usual method, a safe boot, a drive repair and fix permissions, and finally, deleting several plists files in the Library/Preferences folder and also the User/Preferences/ByHost folder. The plist files that I usually delete are the windowserver and the loginwindow, sometimes alf, power management, and several others that the computer will regenerate upon reboot. I have also turned off bluetooth and I use a standard keyboard and mouse. I also have tried different settings with "allow network to wake" (or whatever that is called) and the like. Any one of these things in the past eventually fixes it but I have no idea which one and it isn't working this time.

This time I can't seem to get it fixed and it is very frustrating because as I said earlier, I use the screen as a secondary display and I can't work on it if the screen goes black and does not come on. Just to repeat, once the display goes into sleep, the computer wakes up but the display does not.

Any help would be greatly appreciated because I cannot get it fixed this time.

Thanks,

-Matt