with 2160p@50 or AUTO ? with different HDMI input port?
I did try different hdmi ports and 2160p50 ... same problem (there is no Auto mode)
The problem occurs even in other resolutions like 1080p and 720 so it's not a uhd problem !

There is some pictures of the problem : http://imgur.com/a/jwflr
Edit : the solution I found is to never restart the box when it's working ...
Edit : I installed the 1.0.8 firmware with a ntfs usb drive, is this the problem ?