The issues you have described can be caused by using a low quality displayport cable. Typically the cables bundled with displays are not always of the highest quality. I would recommend looking at a high quality, displayport 1.2 certified cable. More information available here.
Example of a high quality DP1.2 certified cable here.
Ok thanks a lot! I will try it of course as soon as possible!
I will let you know if it solves the problem or not.