iMac 27" won't boot!

Recently my iMac 27" (2011) stopped working. When I turn it on, I hear the little bling-tone and the apple logo and the bar appears. Then it loads a bit and as soon as the iMac is about half way through it suddenly shuts down and - after a few seconds - reboots. Please help! I really don't know what to do. At first I thought it could be the hard drive, so I created a bootable USB-stick and connected it to my iMac. Then I booted with the stick and - the same results: after the iMac is half way through the starting-process it suddenly shuts down.

Then I've installed OS X 10.11 on an external hard-drive and connected it to the iMac - again: same results.

I've already spent HOURS on the internet, looking for a possible solution to this problem. I also tried every possible start-up option, like safe-start, AHT and so on.

I'm kind of desperate right now, since my iMac was really working just fine until now.