I have the same problem. The tv flashes when it overheats but it only overheats when the roku is plugged into the tv. Any idea? It's a Samsung plasma 40" also.
Do you mean the Roku is connected to a USB port on the TV for power? Many TVs don't offer enough power in their USB ports to adaquitely power a Roku Stick. Try using the wall adapter that came with the Roku. If that's not what you meant, then please explain further. If the Roku isn't connected to a TV, how do you know it's overheating?
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.