P2715Q 4K resolution missing. Change Dynamic Range to Full

Hello
A couple weeks ago I purchased the P2715Q monitor and today it finally arrived.

Once I turned on my computer, it automatically recognized the monitor and had the 4k resolution working just by plugging it in basically.

After a couple minutes the monitor went to "power save mode" (who knows why! because I was surfing on the net) and couldn't get it back on.

I rebooted the computer and the screen was on 4k resolution as before. Tried testing the resolution on an old game and at the beginning everything was fine. Minutes after, the screen went black, had to close the game (by pressing a console key - couldn't see anything). The O.S. was black too and again...I rebooted the computer! (message on monitor "power save mode")

The second time I rebooted, the display didn't come with the 4k resolution, now the recommended and maximum resolution is FULLHD (1920x1080).

My first thought was that the driver got corrupted. I uninstalled the driver via device manager, downloaded the newest drivers for my video card and monitor but nothing!

I cannot see the 4k resolution :emotion-7:

Specs:
Video card: Gefoce GTX 660; driver version 355.82  (this video card does support 4K resolutions)
OS: Windows 7 Ultimate; 64bit (SP1)
Monitor's drivers I have tested: DELL_P2715Q-MONITOR_A00-00_DRVR_9XV8F.exe; DELL_P2715Q-MONITOR_A00-00_DRVR_7XVDJ.exe
Display port: mDP input

Please help!