I had the same problem. Just about a week ago Unreal either freezed with whole screen turning yellow or computer just rebooted by itself. It happens immidiatly when I press play button.
Tried older UE4, tried updating video drivers, tried reinstalled my machine. Didnt work! Did a memory check which didnt report anything wrong with my RAM.
Anyway swapped places of the memory cards on my motherboard as someone else did. Now it works.
I have 4x8gb of corsair vengence.