Hello Simon, have you gotten an answer to fix your issue? My friend gave me his iMac whose HHD failed. I installed a new one and installed the OS and all, and I see temperatures of 70 and up from the Power Supply. I noticed the heating begins exactly where the power supply is located. I also noticed the fans and heatsink are positioned below the power supply. Blowing air through and cooling it that way. I wonder if the issue is with the cooling system and not the power supply. Usually a power supply if it malfunctions, it just stops providing power. If the power supply is not being cooled properly, it will heat up continuously. To test the cooling sink I am afraid I will need to take the entire thing apart. I've been trying to avoid this, and searching to see if someone has found a different solution.