My Mac Repeatedly has spontaneously shut down.

This happened once a few days ago, then twice within ten minutes today. I have been using my Mac normally as ever, then several times it has quit everything in the middle of doing something and shut down. Every time this has happened, the computer was plugged in and had a decent charge to begin with. I searched online and found some old tips, I have checked on my battery status and everything looks fine. Today, I installed whatever the latest update from the App Store was for the computer's operating system. Twice tonight, I was watching Hulu and it had been active for about five minutes. The computer suddenly shut off. I powered it back up, and it returned me to what I last was doing. I bought this in Spring 2012, and it's been in fine condition ever since. No problems other than this.

What do I do to stop this? It's insanely annoying!