This sounds like a hardware fault on the power board.
Before looking to get the unit shipped off to a repair centre, we need to rule out issues with rouge remote signal, software issue or other hardware devices plugged into the TV.
When it turns itself off does the standby light flash and then the TV come back on within a few seconds, or does it stay off for a few minutes then come on for no reason?
If the fault occurs regularly I would suggest you disconnect all HDMI and USB cables, and see if the fault clear if it still occurs, then proceed to perform a factory reset via the TV menu.
Should the factory reset not stop the TV powering on and off by itself, I would then suggest you need to get someone to test the power board, as its likely the TV is losing power or not able to maintain a constant energy level, at which point if you provide your postcode I can recommend a local service centre.