Try it with the cover off and a fan pointing at the open case.
I have an MSI laptop with an Intel i7 7th gen CPU and Nvidia 1070 graphics card. I bought the computer used, but figured it would not have a problem running a lot of games. While running Skyrim, it runs fine for a couple of hours, but eventually starts lagging persistently before shutting off completely. The same thing happens for Overwatch, except it will last maybe 30 mins before shutting off.. If anyone has any tips, it would be very much appreciated!