Are you sure the unit is actually shutting off or is it just that the screen goes dark? I know it's hard to tell, but sometimes if you hold a bright flashlight to the screen, you can still see part of the displayed image.
I would recommend as a first test that you go into display settings and turn off Auto Brightness if it is enabled to see if that makes any difference.
If that doesn't help, then I'd take a look at the Event log to see if there are any related entries in there.
Good luck!