Often, this is a symptom of not having a good quality, HDMI 2.0-compliant HDMI cable...
This message was posted on behalf of Intel Corporation
Thank you very much to N. Scott Pearson for the suggestion provided previously.
Besides trying to test the NUC with a high quality HDMI cable, also for testing purposes please test the NUC without the A/V receiver, connect it directly to the TV, just to rule out a possible compatibility problem with it.
By using the HDMI port you should be able to get 2560x1600@60Hz as max resolution, as you can confirm on the following link:
Just in case the processor on the NUC works with Intel® Iris™ Graphics 6100 controller, on the link below you will find the latest version of the graphics driver for it:
You can also try the beta driver as well:
Any questions, please let me know.