iMac loads until a certain point, then screen goes flickering black.

Hello!

My iMac 27", which is no more than max 3 years old, has gotten a new annoying habit. I cannot get access to it at all, due to the screen going black with flickers of color appearing at various parts of the screen. Now this happens at the exact same point of loading every time I start it up.

The screen does work, as I hear the startup sound, I see the Apple-logo and the bar loading. And then it just turns black at the exact same time.

Please help?