I have a problem with yet another 2011 iMac. This time the screen intermittently goes black. The computer is running fine in the background but the screen just out of nowhere goes black. I have tried everything I know... and have gone so far to have the blue shirts run diagnostics and tests over the course of a couple of days. Of course...over that time, it worked fine and the intermittent problem did not expose itself. To complicate matters, the iMac falls under the video card replacement program. But... without the problem showing up during a test, the blue shirts can't replace it. Any ideas?