So I bought the LG 5K display and was delivered two weeks ago. I was having some of the same issues reported here, mainly the flickering, etc. As soon as I installed it, it started flickering, or was completely black, no image, sometimes I was able to see my desktop for 5 seconds, and then it would go black again. I went to an Apple Store, they said this was the first time they heard about these problems, and so they gladly accepted my return. A week later, I purchased another one. Same issues. I went to the store, but this time I asked the guy if I could install it in the store, and test it with any MacBook Pro on display. After talking to a manager, he helped me set it up. The display worked flawlessly. I tried everything, and I couldn't make it fail. Tested moving the cables, using their cables, everything, but the display was rock solid. I decided to keep it, and so went back home. I installed it temporary in a different location on my desk, and to my surprise it worked. I used it for an hour, it worked flawlessly. Then I decided to move out my Thunderbolt Display, and install the LG properly. To my surprise, it didn't work. I wasn't able to get an image, or was flickering like crazy. I moved it back again to where it was, and lo and behold, it worked again. I couldn't grasp why it was failing, and I was dumbfounded to find out the issue was my Wifi routers. Of course is all electronics, but I couldn't imagine this would be an issue, I wrongly supposed these devices have shields against this type of interference. The Thunderbolt Display has an aluminum case, so I can only surmise this is the reason I didn't have any issues whatsoever.
Here's the proof:
Hopefully this will help anyone having this same issue.