How to stop random shut offs?

Acer Chromebook 15, CB3 531 C4A5 shuts off after about 15 minutes of use. Must wait a few minutes before turning it on. It then works for 10 to 15 min. and shuts off again. The dust was cleaned out of it in January because the keyboard was replaced. Since then it has been used with a cooling pad with a fan to prevent over heating. No hot spots could be felt anywhere on the laptop. Unable to find a way to reload the operating system.