iMac Desktop randomly shutting down w Yosemite OS X

iMac is randomly shutting down with Yosemite OS X, I can find solutions for Macbook's with this problem but not for iMac desktop. We have taken the computer to the Genius Bar and they said they ran tests, rebooted the SMC and PRAM. The computer is still randomly shutting down. How can I know if the problem is hardware or if its the power source?