iMac goes to sleep when watching YouTube

Exactly what it says in the title.

I found something interesting to watch on you tube, few moments later my iMac turns off the screen and mutes the sound. Sometimes takes a moment to wake up again if I press a few keys or shake the mouse. Sometimes just crashes and I have to wait until restarts completely.

Note this happens only when you tube is active, even when there are no other apps running. I even have a Turbo Ultra-Cooler at the back of my iMac to prevent overheating. This fan cuts the temperature in half (at least the external one)

These are my Energy saver settings: