I can't say why it might cause overheating, but I'd try the power adapter that came with the Express rather than using the USB port on your TV.
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.
I just bought a Roku streaming stick this week. Received it Wednesday. Thursday I kept getting the message on my tv that it was overheating within the first hour of usage. I checked the recommendations. It is plugged into the back of my tv, a good 8 inches away from the wall. I don’t have it near anything else. I have had the tv off all day and unplugged the stick to take a look at it, and it was very warm. The metal part of it was hot. I did just order the extension, however I am concerned over how warm it is even when everything is turned off. Should there be cause for concern?