Check the follwing:
-Does RO Standard work?
-Set back the RO HQ settings - if you have modified them - to standard settings (C:\Users\YourUserID\AppData\Roaming\J River\Media Center 22\Plugins\madvr\restore default settings or delete the content of the madVR directory and on the next start of MC it will reload the lib
-What are your DIsplay Settings (Tools->Options->Display Settings); I use custom settings (e.g. Film (23.976 fps): 3440x1440 -32 bit@24 Hz etc.?
-Ceck if Hardware accelarate video decoding is aneabled.
-Enable Video Clock (with MC this is better than to let do it in madVR)
- What scaling Algorithm do you use. If you use NEEDI3 its reported in various forums that it leads to problems with the current AMD drivers.
I have GTX 960 with i3-6100 and a LG 4k using Jinc for Chroma Upscaling; LZ3+AR for Image Upscaling and CR+AR+LL for Image downscaling. GPU Utilisation is between 20% - 60% dependent on the content. For higher scaling algorithm the GTX 960 is not capable to upscale all content type e.g. 1080i etc.
If you want to test how true 4k behaves, downlod test clips from:
http://www.4ktv.de/testvideos/