list your pc specs please. run memtest86 to see if your ram is getting errors. if that is the case then that is probably the cause of your shutdowns. but like h4x3r said it is probably overheating. the classic method of testing for overheating is to open up the side panel and have a desktop/pedestal fan blow on the components in your pc. normally if it is overheating it will provide enough airflow to stop the symptoms, thus pointing to overheating as the cause. however if the reboots still occur it is possible that it is something else.