My laptop keeps turning off on it's own for no real reason. At first I thought it was overheating but it's never really that hot. Then once it has turned off I have to wait a few minutes, take the battery out then try again. It's only two years old and has hardly anything on it, just a few pictures and assignments.
Have you checked the power saving options? They might have been set to a shorter period of time if you work with both AC power and a battery.
Although you can't feel that your laptop has got overheated, it still might be overheating it's videochip. You might want to try to diagnose it with third party software to see its temperature. You should try to check CPU temperature also. If you have the option to bring your laptop to a technician tell him/her to check the heat sink.