Display switches to Notebook screen when playing video files

[ultrah]

I am using a HP 3005pr with a Samsung 700Z3A running Windows 8, connected to an Asus PB278Q via DisplayPort

If I play videos on the external monitor (for example embedded youtube) the screen goes black after a few seconds and the picture switches to the notebook screen. After a few seconds it switches back to the external monitor, back to the notebook again and so forth.This issue also occurs at random when not playing videos.

Any advice ?