black screen

When I try to start my game every first time after turning my pc on, the game won't start (or it needs a very long time) and i just get a black screen with nothing on it, the second time i try to start the game it still takes time but it always starts. Is there a fix available somewhere or something or does everyone have the same problem?

the second thing is, when I'm playing online and I minimalise the game (using ALT+TAB) and trying to maximise it again, then I get sometimes the blackscreen too and then I have only sound.

any suggestions?