I've had the same problem with all the roku products from the stick to the exprex 4k to the Ultra LT to the Ultra
Over time they will either "freeze" or go into buffering mode and the problem is overheating
I have moved these products away from other electronics and in area where there is plenty of air circulation.
At first when they are new they work fine and as they get older they start to act up with either freezing the video or go into buffering mode
( by the way the home WiFi network I have is not the problem)
I did an experiment and the results was remarkable
I put the unit on a small bowl that I filled with ice cubes and the unit (Ultra LT) didn't have a single issue not once did it freeze up or buffer for about 12 hours
the was totally melted but the water was still cool
In conclusion I think Roku could add a low speed fan to sit underneath the Unit to circulate the hot air that's created by the WiFi reception antenna
These units get hot and over time the reception is weakened to the point where they will fail
Thanks for reading
