iMac 27" (late 2009) intermittent black screen

Late 2009 27" iMac just started random screen black-outs within the past few days. Everything still runs in the background and if I put it to sleep and wake it up, the screen brightness is back on. Sometimes it takes only a few seconds before it goes black again, other times it can take hours.

I've reset the SMC / PRAM; unplugged all the USBs; messed with the brightness; basically everything that's been suggested short of replacing the logic board. Disk Utility shows no problems or failures. My next step is to increase the RAM (which I planned to do anyway so I could upgrade to El Capitan) but I don't know if this will do anything in regards to the black screen issue.

Has the actual problem been identified yet? Real solutions??

~ Thanks!!

Just some history: Straight out of the box, my iMac had a flickering screen that no one knew out how to repair. It took months before a firmware update came out that fixed the issue. I almost wonder if it's happening again?

I've seen a LOT of posts and YouTube videos with people having the same black screen problem.

My hard-drive crashed back in 2012 or 2013-ish. Apple generously replaced it at no cost due to a factory defect in my particular model; however, my CD/DVD drive, Front Row and iPhoto all stopped working when they returned it to me (new software had been installed). My magic mouse also stopped working after about 2 years but I don't think that was software related (I replaced it with a USB infrared Logitech which has run strong for 4 years and doesn't go through nearly as many batteries as my magic mouse did).