I have no doubt that you have read the other threads on this issue but, just in case you didn't......
We have over 200 headless servers of this category ie NUC, Mac Mini and now Gigabyte Brix. We have always had problems with Intel's imbedded HD graphics offerings, the first being the HD3000 (including the Mac Mini's). While I would like to think that this is a driver problem, I'm starting to think that the hardware was never designed for headless activity. Very early on we discovered that adding a "VGA dongle" to either the HDMI or DP ports on these devices was a low cost, minimal effort solution to the problem.
I know that this is probably not what you wanted to hear but after many years with this problem, we simply decided to move on with this simple "fix" and make the most of the benefits of this small form factor. The best article I have seen on the building of a VGA Dongle is this one - The Personal Computer - VGA Dummy. All you need is to buy an HDMI to VGA (female) cable or DP to VGA (female) cable on eBay, buy a handful of 75ohm resistors and you are in business.