If the issue isn’t the hard drive, then I would go through the exercise of hard-wiring to the Roku 3 box (to see if it is your wireless setup). I have a 100 foot ethernet cable that I use for this purpose. Just temporarily run it down the hallway/steps in order to get wireless out of the equation.
Your wireless router looks to be good enough for running a Tablo, but, wireless is wireless and there are so many things that can cause interference. For my situation I found a way to run CAT-5 (and later CAT-6) ethernet cables to every room in my house. It was some work, but, I never have issues with buffering. So it was worth the effort.