I have two iMacs a 27 inch late 2009 and a 27 inch late 2014. Initially I thought it was a problem with the 2009 only (I was thinking graphics card). During some random event(clicking a link, going to a web page, opening a document)will lock up the screen. I can move the mouse pointer around but nothing works. When the screen blacks out I can use the mouse or keyboard to get the backlight on but just get a black screen with backlight. I didn't suspect it was related to multiple user accounts until my wife wanted to add kids accounts to the 2014 iMac. After she added some kids accounts she started experiencing the same thing. If a single user is logged into either system it can run without any problems. Once an additional user logs in it is a just a matter of some random event causing lockup of the screen. Then I am forced to do a cold boot. I have had this problem with Mavericks and Yosemite on the 2009 and with Yosemite on the 2014. I haven't updated either system to El Capitan yet.