iMac Crashes all the time

Hello,

I have this problem more than one year and I don't know how to fix it. Even the Apple Support couldn't help me out.

You're my last hope!

The Problem:

What happens?

The imac begins to make noise and then the screen becomes black. If I wait some seconds and tipp on the keyboard, the login screen appears and I can log in again. Some Seconds later it happens again..

When it happens?

It happens very often, when I have a Video (itunes) active and want to play for example Hearthstone at the same time. But it also happens sometimes if I just watch the keynote on Safari. I can force the crash, if I open many games and videos.

What I have tried?

- reinstall (without delting the applications on the disk)

- I changed the ram

- deleted chaches

- Ram reset