My laptop is shutting off for no apparent reason?

I have an HP Envy Ultrabook 4-1025dx. I have had no past issues with it, but today I booted it up, and it worked normally for about 3 minutes, but then it just shut off. I sat shocked for a moment or two and then tried to boot it up again. The laptop turned on; however, the screen remained blank, much like a computer with a bad RAM stick would. Nothing showed up on the screen for several minutes, so I had to force it to shut off. I waited for about 10 or 15 minutes and gave it another shot. This time the laptop booted up fine, and it ran normally for the next couple of hours that I used it. This evening I got on my laptop again, booted it up normally, and it proceeded to do the same thing: run for a few minutes and then shut off unexpectedly. I have no idea what is causing this. My antivirus is up to date and I scan for viruses often; I am certain I did not download any program with malicious software; I clean it software-wise often; I defrag regularly; the problem did not start after any updates; I have had any recent hardware changes; and I have plenty of RAM (12 Gigs). Could this be some sort of overheating issue, and if so why does it run fine for a couple hours at some times and at others only for a few minutes?