Maybe someone can relate. I'm on a 24", Early 2008 iMac, 6GB ram, NVIDIA GeForce 8800 GS 512mb and running OS X 10.9.2 and on Monday something really weird happened... I'm using the computer, browsing, etc, I get up and walk away from it and upon coming back to it, the screen was "dark" It almost seemed like the back-light went 100% off. Not the same type of darkness as putting the screen to sleep or anything. Computer was completely unresponsive, yet, on with the drive spinning. No strange noises. Clicking of the mouse yielded nothing positive or beneficial, so... I did a hard shutdown, restarted, and everything was fine for the rest of the night. Then today, (Tuesday) I'm on the computer and randomly out of the blue the screen dimmed/flickered slightly, and within seconds, I got the same dark screen. I tapped the enclosure but that didn't help. I fiddled the cable in the back that's hooked up to an external TV as a second display and all of a sudden the screen on the iMac came back. Coincidence? Who knows. However, the computer was completely unresponsive. Mouse movements, key bashing does/did nothing. Next up, did a hard shutdown, unplugged the iMac cord, let it sit for a couple minutes before running a 1.5 hour Apple hardware test. Everything came back FINE. Couple of interesting points: I've never used the USTREAM site in the past until this Monday. Both these screen blackouts happened when I had USTREAM streaming video in Firefox. Could this be related to something on that website? Flash based? I wonder? I use Photoshop/Lightroom and frequently play videos on here (YouTube specifically) no issues with ANY of those applications/sites even on the heaviest load. I haven't found that many GPU crash results on google from the use of Ustream, but I have found some related to general Flash based applications and videos. Kernal panics usually result in a complete shut down (in my experience). This was limited to no warning at all, just flickering slightly followed by a dark screen and a completely unresponsive computer. What could of happened here?