gtx 960 loses display configuration

I just bought a geoforce gtx 960 card and installed NVidia driver 975 for the first time on my Ubuntu 14.04 (3.16.0-77-generic). The display is a Visio 40" 4K tv conncected over HDMI.

The problem is that if I turn the display off and on, the screen resolution is switched from 3840x2160 to 900x600, and the display type changes to ‘Unknown’.

The same happens after suspend/resume.

I searched this group but don’t see anything obviuosly similar.

thank you for your help.