Whenever my laptops not on a desk it tends to restart itself

Whenever my laptops not on a desk it tends to restart itself after about a few minutes of use. Its never the same. Sometimes ill be using it for a second on my lap and it will freeze and restart itself other times I use it for an hour and I doesnt do anyhting. I get a message "Your computer restarted because of a problem".When im on a desk it will run flawlessly. Im getting really tired of it. It will freeze first before restarting without warning. Thanks