Hi! I've been having this problem with my 3.5 year old 24-inch aluminum 3.06ghz iMac quite some time now. I have asked this question at 4-5 other Mac/Apple specialized websites but NONE has ever given me a solution or even an answer of what the problem might be. The problem is that, when my iMac is booted up for 5-10mins without performing heavy duty, it shuts itself down without any warning or message. It looks like the power cable gets pulled out. I've done a million things already, I'll list them here for the easy of reading: - Formatted HDD - Full new install of OS X - Bought new RAM and replaced them with the old ones - PRAM, NVRAM,... - Ran Apple Hardware Test many times for hours without any message saying there is a hardware problem - Tested the PSU with a multimeter but everything OK Noticed that the LED diagnostic leds were burning so I searched the meaning of each light. At iFixit I found out that the fourth led indicates that there is a heat issue. I used compressed air to dust out the iMac and the fans but there wasn'te even any small dust particle in there. I also installed Hardware Monitor to see the temperatures and i saw the Heatsink and GPU was around 70°C before it shutter down. So I bought cooling paste and applied on the CPU as well as on the GPU. Then i booted up, saw the 4 leds still were burning. I ran Hardware Monitor to see what the temperature was. All was under or around 20° Celcius and the Heatsink was around 50° Celcius (what i think is a great improvement since previous). Still it fell off just like before. I really don't have any clue what the problem could be. Could someone please give me some advice to try things? Thanks in advance!!