MacBook pro has black screen

Hi,

Since a few months my MacBook has a black screen when it returns from sleep-mode and also some application's don't run anymore, they just keep hanging when you start them up. This problem started a few months ago and the weird thing is that i i brought it to an apple store 2 times and every time i got it back the mac worked fine for about two weeks and than suddenly it appears again with no reason. I also tried to reset the RAM, but that didn't helped anything. Can someone who had it also, please give me advice?
Thanks.

Have you changed ANY internal hardware from the stock? E.g. added memory or changed hard disk.

No i didn't, the problem just suddenly occurred..