crashing macbook air

If it happens every time you do something that's actually good news, in that you may be able to determine the reason and fix it. Restart your Mac in "Safe Mode", and determine if the spontaneous shutdowns persist or subside while in that mode: Use safe mode to isolate issues with your Mac.

Then, restart normally (not Safe Mode) and perform the same action that caused it to shut down.

Does it restart on its own, with a dialog similar to that shown in If your Mac restarted because of a problem?