iPod Nano 7th black screen but it works

Hi to everyone. I've got a problem to be solved... or better, my iPod Nano 7th 16gb has got a problem to be solved🙂

When I connect it to my iMac, iTunes starts and recognize my ipod. I can work with it via iMac, but the ipod screen is black. It doesn't seem to be an hardware problem because on my Apple Store they could restore it, but after some hours the screen went black again. I reinstalled the whole system on my iMac, I formatted my iPod also via Windows, via Os System, I tried from the beginning, but still nothing... the screen is black. Sometimes, when I keep holding both the keys (home and hold), for a while i can see the Apple logo then the black screen again.

I use Mavericks 10.9.4 and iTunes 12.3.1.23

When I connect it to the usb of my stereo car system it works and I can use it by choosing the songs via stereo, but for example if I want to use it while I'm running I can't do because of the black screen.

Thank you for helping me.

Greetings.