have you ever even thought it's the power user?
maybe it goes off because it's unactive
if this is not the problem then it maybe over heating.
put your laptop on a desk flat, leave it turned OFF for 20mins, go back to the laptop, switch it on, get a timer time how long its on for.
when it goes of, leave it for 20mins, now put the laptop in mid air so air gets out, now time it again, if its over 5mins longer then last time it's your heeating