Lumia 640 XL Black Screen issue

[h3rm3s]

Hi there, I'm experiencing an annoying problem with my Lumia 640. After few weeks i bought it, the display gets randomly dark and no interaction with the phone is possible. I can nevertheless receive calls, and the touch function is working, cause I was able to answer the call, remembering where to touch the display, even if it was completely dark. If u look at it being in a dark room, I can see that the display gets slightly illuminated, when I turn it on, even if it keeps being dark. Sometimes, all of a sudden, images, icons, light and colors come back. sometimes just after knocking it on the back a little. I've tried a soft reset but the issue is still present. What do you think? What do you suggest? Thanks in advance for your advice