After about a year, my 50S431 does the same thing. Was finally able to get the set into recovery mode, connect successfully to my wireless router, download update for the TV. It said "Restarting" and then went right back into repeating the initial boot screen.....which was the problem in the first place. This TV is a real POS.
After trying everything listed on every thread, what worked for me was replacing the main board inside the TV. Had my TV for just over a year, so warranty wouldn't cover a tech fixing it. Ordered the main board online using my model number to find the right one and just replaced it. (You can find videos on how to replace it online. You unscrew the back of the TV, unplug the motherboard, unscrew the motherboard and then replace it!) Worked like a charm!