I was unlucky to have this sudden failure with my Mac Pro. Since this afternoon my workhorse computer is booting with these strange glitches on the screen.

Update
Helpful DIY tip of baking my computer's graphics card seemed to work. The Apple discussion forum was very helpful. I was glad to be offered a solution I tried and it worked.

Wonders of baking! I baked the card at Gas Mark 5 about 9 min. After it cooled down I dolloped little bit of cream and raspberry jam over it. Voila, perfect for my cream tea :-D

Seriously! It was an easy repair and I am very happy that my Mac Pro is up and running again.