iMac freezing with graphical artifacts, what to do? My iMac 27" from 2011 has recently been experiencing random freezes. It either restarts or I am forced to reboot (holding power button several seconds). When frozen the iMac does not respond to pings and nothing at all is written to the system logs.

Symptoms are graphics issues which happens just before it freezes. Tearing, flickering and random graphical artifacts. When it freezes, it usually shows a screen with striped patterns, a grey or black screen. I have checked CPU and GPU temperature and there are no problems with overheating.

Once it hangs and I reboot, the computer usually fails to boot and hangs during boot with a gray screen. At this point I cannot get it to boot at all, except into single (console) mode. Checking disks reveals nothing and running apples hardware tests tells me everything is ok.

If I leave the computer turned off for a while (say over the night) and then try to boot it, everything works. 

Someone suggested it may be power-related somehow, but I have no idea how to check that.

Any ideas how to diagnose this?
Turned out to be a faulty graphics card. I replaced it with a new, and problems went away.