Sounds like a faulty unit. Would be a good test to try a NEW AC3100 out of the box and see if the same thing happens.
Decision made. I wanted to share with the thread that I returned the Netgear R8000 and purchased a Asus RT-AC3100 (HW Ver A5). Installed and working well on merlin 382.1_beta2 firmware. The router is sitting on one of those closet-wire shelves. Because of all the posts of the AC3100 running hot (78 to 82 degree C) and my experience with the flakey refurb unit, I decided to zip-tie a Thermaltake USB Fan underneath it (plugged into wall-powered USB surge & power adapter; fan speed at it's lowest setting). Temp avg is normally in the mid-60s but it's 54C right now as I type this. I'm very happy with this router and merlin firmware combo. Best I've ever owned.