Mid-2007 iMac 27” running El Capitan. Started going to black screen but running. Tried all the start-up options holding various key combinations to no avail. Would run for about 10 minutes then black screen. Had to hard reboot each time. Read something about SMC Fan Control. Downloaded it and could see the temp was running quite high. Adjusted the options for the three controllers and fans to set the fan speed at its highest. Of course now it’s quite loud but not bad. Anyway, it fixed it. The machine was just getting too hot and shutting the display off. Have been running the machine with fans on high now for past 24 hours and no black screens. Get the app! It’s free but provides for donations. Gave them $12 this morning for it. Support your local developers :-)