I think I solved my own problem. Although the subscription comes with 3 connections I am currently using a Trial having only 1 connection. Once my full subscription becomes active (having multiple connections) if the problem continues I will post a Reply here. Otherwise, hopefully other users...
Hello, I am using a new IPTV service with 3 connections. The service runs flawlessly on both my Shield and M8S Pro L devices when running just one of them at a time, but immediately begins intermittent "payback error" (buffering) on both devices once I begin streaming using a second connection...