If you have 30 minute window to find the problem go to the event viewer and see what error code comes up when the machine shuts down . Knowing the exact time of the shut down will make it easy to find the event that lead to the shutdown. Are there any errors coming up on the screen before your laptop shuts down ?
BSOD? If the event viewer brings no joy you can open the laptop a and visually inspect the motherboard for possible blown capacitors . It may be a thermal issue . Clean your fan and heat sink . You may also want to reapply your thermal paste. Hope this helps