I have recently purchased 2 x Roku express boxes. One connected to a Bush TV, which works fine. One connected to a Samsung TV, which the Roku device continually restarts. Having two devices enabled me to ‘divide and conquer’ and swap things around to identify what the problem was. I identified the problem to be with the USB voltage output of the Samsung TV.
Instead of plugging the USB cable to the Samsung TV I plugged it into a mains USB adapter, which fixed the problem. Not sure this helps anyone facing similar issues, but it worked for me 🐵