Hi Ollie. Welcome to justanswer.
Remove all the connections from the Tv. Remove it from the power outlet. Let it sit for 10 minutes and then put all the connections back and see if it still turns off or not.
Also tell me when the TV turns off by itsef, does the light on the TV goes off or does it still glows?
This means that the main board is failing due to any internal fault or any component on the main board has gone bad.
It is probably the very common problem with the Samsung capacitors on the power board which is not holding charge. The main board needs to be replaced.
Was there any thunderstorm in area which might have caused this fault?
Is the Tv still under warranty?
Ollie, do you have any query?