My iMac has been working like a charm all the time except a few months back it started to reveal symptoms on each boot. I keep iMac turned on all the time, so it hasn't bothered me much, only on software update when you have to reboot the machine. Every time I had to boot iMac it hangs on the grey start-up screen with Apple logo and spinning wheel. I can get through it pressing C while booting (C is the key to boot from external device, such as DVD or USB) without having any discs inserted or devices connected. Yesterday I installed the 10.8.4 Combo update and booted with Cmd-V to access the verbose boot mode. I managed to get there 3 times in a row, every time it freezed on the line: AppleIntelCPUPowerManagement Two times only partly printed "AppleIntelCPUPowerM". Now I'm stuck with a black screen and no start-up chime. I can hear the fans are spinning slowly and the hard drive and the optical drive both at least get's power. No other marks of life here. Did the SMC reset - nothing. No response on keyboard (tried to boot to safe mode, make a PRAM reset, boot from external device.. everything). Then I tried to remove all the RAM banks and boot. One beep constantly repeating with a delay of a second which means "No RAM installed". Then I started to put in one bank at a time and some of them got three beeps ("No RAM banks passed memory testing"). Then I started to put multiple banks at the same time, on some booting trials the same RAM bank was resulted as faulty, on some trials as functional. Based on this information from the verbose boot and the experience with RAM, is it.. a) the power supply (because it froze on AppleIntelCPUPowerManagement) b) the logic board (same RAM banks are faulty and functional on different boots) c) OS X kernel (while AppleIntelCPUPowerManagement is a kext) or something else? Is there anything I can do? Do I have to take it to the Apple repair service? If it's the logic board, can I ask for a newer replacement board, let's say, I want to switch the mid-2011 logic board to an early-2013 logic board, is it possible?